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Caktus GroupManaging your AWS Container Infrastructure with Python

We deploy Python/Django apps to a wide variety of hosting providers at Caktus. Our django-project-template includes a Salt configuration to set up an Ubuntu virtual machine on just about any hosting provider, from scratch. We've also modified this a number of times for local hosting requirements when our customer required the application we built to be hosted on hardware they control. In the past, we also built our own tool for creating and managing EC2 instances automatically via the Amazon Web Services (AWS) APIs. In March, my colleague Dan Poirier wrote an excellent post about deploying Django applications to Elastic Beanstalk demonstrating how we’ve used that service.

AWS have added many managed services that help ease the process of hosting web applications on AWS. The most important addition to the AWS stack (for us) was undoubtedly Amazon RDS for Postgres, launched in November 2013. As long-time advocates for Postgres, this addition to the AWS suite was the final puzzle piece necessary for building an AWS infrastructure for a typical Django app that requires little to no manual management. Still, the suite of AWS tools and services is immense, and configuring these manually is time-consuming and error-prone; despite everything it offers, setting up "one-click" deploys to AWS (à la Heroku) is still a complex challenge.

In this post, I'll be discussing another approach to hosting Python/Django apps and managing server infrastructure on AWS. In particular, we'll be looking at a Python library called troposphere that allows you to describe AWS resources using Python and generate CloudFormation templates to upload to AWS. We'll also look at a sample collection of troposphere scripts I compiled as part of the preparation for this post, which I've named (at least for now) AWS Container Basics.

Introduction to CloudFormation and Troposphere

CloudFormation is Amazon's answer to automated resource provisioning. A CloudFormation template is simply a JSON file that describes AWS resources and the relationships between them. It allows you to define Parameters (inputs) to the template and even includes a small set of intrinsic functions for more complex use cases. Relationships between resources are defined using the Ref function.

Troposphere allows you to accomplish all of the same things, but with the added benefit of writing Python code rather than JSON. To give you an idea of how Troposphere works, here's a quick example that creates an S3 bucket for hosting (public) static assets for your application (e.g., in the event you wanted to host your Django static media on S3):

from troposphere import Join, Template
from troposphere.s3 import (

template = Template()
domain_name = "myapp.com"

                AllowedOrigins=[Join("", ["https://", domain_name])],
                AllowedMethods=["POST", "PUT", "HEAD", "GET"],


This generates a JSON dump that looks very similar to the corresponding Python code, which can be uploaded to CloudFormation to create and manage this S3 bucket. Why not just write this directly in JSON, one might ask? The advantages to using Troposphere are that:

  1. it gives you all the power of Python to describe or create resources conditionally (e.g., to easily provide multiple versions of the same template),
  2. it provides compile-time detection of naming or syntax errors, e.g., via flake8 or Python itself, and
  3. it also validates (most of) the structure of a template, e.g., ensuring that the correct object types are provided when creating a resource.

Troposphere does not detect all possible errors you might encounter when building a template for CloudFormation, but it does significantly improve one's ability to detect and fix errors quickly, without the need to upload the template to CloudFormation for a live test.

Supported resources

Creating an S3 bucket is a simple example, and you don't really need Troposphere to do that. How does this scale to larger, more complex infrastructure requirements?

As of the time of this post, Troposphere includes support for 39 different resource types (such as EC2, ECS, RDS, and Elastic Beanstalk). Perhaps most importantly, within its EC2 package, Troposphere includes support for creating VPCs, subnets, routes, and related network infrastructure. This means you can easily create a template for a VPC that is split across availability zones, and then programmatically define resources inside those subnets/VPCs. A stack for hosting an entire, self-contained application can be templated and easily duplicated for different application environments such as staging and production.

AWS managed services for a typical web app

AWS includes a wide array of managed services. Beyond EC2, what are some of the services one might need to host a Dockerized web application on AWS? Although each application is unique and will have differing managed service needs, some of the services one is likely to encounter when hosting a Python/Django (or any other) web application on AWS are:

  • S3 for storing and serving static and/or uploaded media
  • RDS for a Postgres (or MySQL) database
  • ElastiCache, which supports both Memcached and Redis, for a cache, session store, and/or message broker
  • CloudFront, which provides edge servers for faster serving of static resources
  • Certificate Manager, which provides a free SSL certificate for your AWS-provided load balancer and supports automatic renewal
  • Virtual Private Clouds (VPCs) for overall network management
  • Elastic Load Balancers (ELBs), which allow you to transparently spread traffic across Availability Zones (AZs). These are managed by AWS and the underlying IPs may change over time.

Provisioning your application servers

For hosting a Python/Django application on AWS, you have essentially four options:

  • Configure your application as a set of task definitions and/or services using the AWS Elastic Container Service (ECS). This is a complex service, and I don't recommend it as a starting point.
  • Create an Elastic Beanstalk Multicontainer Docker environment (which actually creates and manages an ECS Cluster for you behind the scenes). This provides much of the flexibility of ECS, but decouples the deployment and container definitions from the infrastructure. This makes it easier to set up your infrastructure once and be confident that you can continue to use it as your requirements for running additional tasks (e.g., background tasks via Celery) change over the lifetime of a project.
  • Configure an array of EC2 instances yourself, either by creating an AMI of your application or manually configuring EC2 instances with Salt, Ansible, Chef, Puppet, or another such tool. This is an option that facilitates migration for legacy applications that might already have all the tools in place to provision application servers, and it's typically fairly simple to modify these setups to point your application configuration to external database and cache servers. This is the only option available for projects using AWS GovCloud, which at the time of this post supports neither ECS nor EB.
  • Create an Elastic Beanstalk Python environment. This option is similar to configuring an array of EC2 instances yourself, but AWS manages provisioning the servers for you, based on the instructions you provide. This is the approach described in Dan's blog post on Amazon Elastic Beanstalk.

Putting it all together

This was originally a hobby / weekend learning project for me. I'm much indebted to the blog post by Jean-Philippe Serafin (no relation to Caktus) titled How to build a scalable AWS web app stack using ECS and CloudFormation, which I recommend reading to see how one can construct a comprehensive set of managed AWS resources in a single CloudFormation stack. Rather than repeat all of that here, however, I'm going to focus on some of the outcomes and potential uses for this project.

Jean-Philippe Serafin provided all the code for his blog post on GitHub. Starting from that, I've updated and released another project -- a workable solution for hosting fully-featured Python/Django apps, relying entirely on AWS managed services -- on GitHub under the name AWS Container Basics. It includes several configuration variants (thanks to Troposphere) that support stacks with and without NAT gateways as well as three of the application server hosting options outlined above (ECS, EB Multicontainer Docker, or EC2). Contributions are also welcome!

Setting up a demo

To learn more about how AWS works, I recommend creating a stack of your own to play with. You can do so for free if you have an account that's still within the 12-month free tier . If you don't have an account or it's past its free tier window, you can create a new account at aws.amazon.com (AWS does not frown on individuals or companies having multiple accounts, in fact, it's encouraged as an approach for keeping different applications or even environments properly isolated). Once you have an account ready:

  • Make sure you have your preferred region selected in the console via the menu in the top right corner. Sometimes AWS selects an unintuitive default, even after you have resources created in another region.

  • If you haven't already, you'll need to upload your SSH public key to EC2 (or create a new key pair). You can do so from the Key Pairs section of the EC2 Console.

  • Next, click the button below to launch a new stack:

  • On the Select Template page:

  • On the Specify Details page:

    • Enter a Stack Name of your choosing. Names that can be distinguished via the first 5 characters are better, because the name will be trimmed when generating names for the underlying AWS resources.
    • Change the instance types if you wish, however, note that the t2.micro instance type is available within the AWS free tier for EC2, RDS, and ElastiCache.
    • Enter a DatabaseEngineVersion. I recommend using the latest version of Postgres supported by RDS. As of the time of this post, that is 9.6.2
    • Generate and add a random DatabasePassword for RDS. While the stack is configured to pass this to your application automatically (via DATABASE_URL), RDS and CloudFormation do not support generating their own passwords at this time.
    • Enter a DomainName. This should be the fully-qualified domain name, e.g., myapp.mydomain.com. Your email address (or one you have access to) should be listed in the Whois database for the domain. The domain name will be used for several things, including generation of a free SSL certificate via the AWS Certificate Manager. When you create the stack, you will receive an email asking you to approve the certificate (which you must do before the stack will finish creating). The DNS for this domain doesn't need to exist yet (you'll update this later).
    • For the KeyName, select the key you created or uploaded in the prior step.
    • For the SecretKey, generate a random SECRET_KEY which will be added to the environment (for use by Django, if needed). If your application doesn't need a SECRET_KEY, enter a dummy value here. This can be changed later, if needed.
    • Once you're happy with the values, click Next.
  • On the Options page, click Next (no additional tags, permissions, or notifications are necessary, so these can all be left blank).

  • On the Review page, double check that everything is correct, check the "I acknowledge that AWS CloudFormation might create IAM resources." box, and click Create.

The stack will take about 30 minutes to create, and you can monitor its progress by selecting the stack on the CloudFormation Stacks page and monitoring the Resources and/or Events tabs.

Using the demo

When it is finished, you'll have an Elastic Beanstalk Multicontainer Docker environment running inside a dedicated VPC, along with an S3 bucket for static assets (including an associated CloudFront distribution), a private S3 bucket for uploaded media, a Postgres database, and a Redis instance for caching, session storage, and/or use as a task broker. The environment variables provided to your container are as follows:

  • AWS_STORAGE_BUCKET_NAME: The name of the S3 bucket in which your application should store static assets.
  • AWS_PRIVATE_STORAGE_BUCKET_NAME: The name of the S3 bucket in which your application should store private/uploaded files or media (make sure you configure your storage backend to require authentication to read objects and encrypt them at rest, if needed).
  • CDN_DOMAIN_NAME: The domain name of the CloudFront distribution connected to the above S3 bucket; you should use this (or the S3 bucket URL directly) to refer to static assets in your HTML.
  • DOMAIN_NAME: The domain name you specified when creating the stack, which will be associated with the automatically-generated SSL certificate.
  • SECRET_KEY: The secret key you specified when creating this stack
  • DATABASE_URL: The URL to the RDS instance created as part of this stack.
  • REDIS_URL: The URL to the Redis instance created as part of this stack (may be used as a cache or session storage, e.g.). Note that Redis supports multiple databases and no database ID is included as part of the URL, so you should append a forward slash and the integer index of the database, e.g., /0.

Optional: Uploading your Docker image to the EC2 Container Registry

One of the AWS resources created by AWS Container Basics is an EC2 Container Registry (ECR) repository. If you're using Docker and don't have a place to store images already (or would prefer to consolidate hosting at AWS to simplify authentication), you can push your Docker image to ECR. You can build and push your Docker image as follows:

DOCKER_TAG=$(git rev-parse HEAD)  # or "latest", if you prefer
$(aws ecr get-login --region <region>)
docker build -t <stack-name> .
docker tag <stack-name>:$DOCKER_TAG <account-id>.dkr.ecr.<region>.amazonaws.com/<stack-name>:$DOCKER_TAG
docker push <account-id>.dkr.ecr.<region>.amazonaws.com/<stack-name>:$DOCKER_TAG

You will need to replace <stack-name> with the name of the stack you entered above, <account-id> with your AWS Account ID, and <region> with your AWS region. You can also see these commands with the appropriate variables filled in by clicking the "View Push Commands" button on the Amazon ECS Repository detail page in the AWS console (note that AWS defaults to using a DOCKER_TAG of latest instead of using the Git commit SHA).

Updating existing stacks

CloudFormation, and by extension Troposphere, also support the concept of "updating" existing stacks. This means you can take an existing CloudFormation template such as AWS Container Basics, fork and tweak it to your needs, and upload the new template to CloudFormation. CloudFormation will calculate the minimum changes necessary to implement the change, inform you of what those are, and give you the option to proceed or decline. Some changes can be done as modifications whereas other, more significant changes (such as enabling encryption on an RDS instance or changing the solution stack for an Elastic Beanstalk environment) require destroying and recreating the underlying resource. CloudFormation will inform you if it needs to do this, so inspect the proposed change list carefully.

Coming Soon: Deployment

In the next post, I'll go over several options for deploying to your newly created stack. In the meantime, the AWS Container Basics README describes one simple option.

Og MacielOn Reading and writing

Picture of 'On Writing'

This week I started reading On Writing: A Memoir of the Craft by Stephen King, a book that has been mentioned a few times by people I usually interview for my weekly podcast as something that is both inspiring and has had a major impact on their lives and careers. After the third or forth time someone mentioned I finally broke down and got myself a copy at the local bookstore.

I have to say that, so far, I am completely blown away by this book! I can totally see why everyone else recommended it as something that people should add to their BTR (Books To Read) list! First of all, the first section of the book, which Stephen King calls his 'C.V.' (and not his memories or auto biography), covers his early life as a child, his experiences and struggles (there are quite a few passages that will most likely get you to laugh out loud) growing up with his mom and older brother, Dan. This section, roughly speaking around 100 pages or so, are so easy to relate to that you can probably be done with them in about 2 hours no matter what your reading pace is. I am always captivated to learn how someone 'came to be', the real 'behind the scenes' if you will, of how someone started out their lives and the paths they took to get to where they are now.

The next sections talk about what any aspiring writer should add to their 'toolbox' and it covers many interesting topics and suggestions which, if you really think about it, makes a ton of sense. This is where I am in the book right now, and though it isn't as captivating as the first section, it should still appeal to anyone looking for solid advice on how to become a better writer in my humble opinion.

Though I one day do aspire to become a published writer (fiction most likely), and I am enjoying this book that I'm having a real hard time putting it down, the reason why I chose to write about it is related to a piece of advice that Stephen King shares with the reader about the habit of reading.

Stephen King claims that, to become a better writer one must at least obey the following rules:

  • Read every day!
  • Write every day!

It is by reading a lot (something that should come naturally to anyone who reads every day) that one learns new vocabulary words, different styles of prose, how to structure ideas into paragraphs and rhythm. He says that it doesn't matter if you read in 'tiny sips' or in huge 'swallows', but as long as you continue to read every day, you'll develop a great and, in his opinion, required habit for becoming a better writer. Obviously, based on his two rules you'd need to write every day too, and if you're one of us who is toying with the idea of becoming a writer one day (or want to become a better writer), I too highly recommend that you give this book a shot! I know, I know, I have not finished it yet but still... I highly recommend it!

Back to the habit of reading and the purpose of this post, I remember back in 2008 my own 'struggle' to 'find the time' to read non technical books. You know, reading for fun? Back then I was doing a lot of reading, but mostly it consisted of blog posts and articles recommended by my RSS feeds, and since I was very much involved with a lot of different open source projects, I mostly read about GNOME, KDE, Ubuntu and Python. Just the thought of reading a book that did not cover any of these topics gave me a feeling of uneasiness and I couldn't picture myself dedicating time, precious time, to reading 'for fun.' But eventually I realized that I needed to add a bit more variety to my reading experience and that sitting in front of my computer during my lunch break would not help me with this at all. There were too many distractions to lure me away from any book I may be trying to read.

I started out by picking up a book that everyone around me had mentioned many times as being 'wicked cool' and 'couldn't put it down' kind of book. Back then I worked at a startup and most of the engineers around me were much younger than me and at one point or another most of them were into 'the new Harry Potter' book. I confess that I felt judgmental and couldn't fathom the idea of reading a 'kid book' but since I was trying to create a new habit and since my previous attempts had failed miserably, I figured that something drastic was just what the doctor would have recommended. One day after work, before driving back home, I stopped by the public library and picked up Harry Potter and the Sorcerer's Stone.

Next day at work when I took my lunch break, I locked my laptop and went downstairs to a quiet corner of the building's lobby. I picked a nice, comfortable seat with a lot of natural sun light and view of the main entrance and started reading... or at least I thought I did. Whenever I started to read a paragraph, someone would open the door at the main entrance to the building either on their way in or out, and with them went my focus and my mind would start wandering. Eventually I'd catch myself and back to the book my eyes went, only to be disrupted by the next person opening the door. Needless to say, experiment 'Get More Reading Done' was an utter failure!

Caktus Group5 Ways to Deploy Your Python Web App in 2017 (PyCon 2017 Must-See Talk 4/6)

Part four of six in the 2017 edition of our annual PyCon Must-See Series, highlighting the talks our staff especially loved at PyCon. While there were many great talks, this is our team's shortlist.

I went into Andrew T Baker’s talk on deploying Python applications with some excitement about learning some new deployment methods. I had no idea that Andrew was going to deploy a simple “Hello World” app live, in 5 different ways!

  1. First up, Andrew used ngrok to expose localhost running on his machine to the web. I’ve used ngrok before to share with QA, but never thought about using it to share with a client. Interesting!

  2. Heroku was up next with a gunicorn Python web app server, with a warning that scaling is costly after the one free app per account.

  3. The third deploy was “Serverless” with an example via AWS Lambda, although many other serverless options exist.

  4. The next deploy was described as the way most web shops deploy, via virtual machines. The example deploy was done over the Google Cloud Platform, but another popular method for this is via Amazon EC2. This method is fairly manual, Andrew explained, with a need to Secure Shell (SSH) into your server after you spin it up.

  5. The final deploy was done via Docker with a warning that it is still fairly new and there isn't as much documentation available.

I am planning to rewatch Andrew’s talk and follow along on my machine. I’m excited to see what I can do.

Tim HopperPython Plotting for Exploratory Data Analysis

Plotting is an essential component of data analysis. As a data scientist, I spend a significant amount of my time making simple plots to understand complex data sets (exploratory data analysis) and help others understand them (presentations).

In particular, I make a lot of bar charts (including histograms), line plots (including time series), scatter plots, and density plots from data in Pandas data frames. I often want to facet these on various categorical variables and layer them on a common grid.

To that end, I made pythonplot.com, a brief introduction to Python plotting libraries and a "rosetta stone" comparing how to use them. I also included comparison to ggplot2, the R plotting library that I and many others consider a gold standard.

Philip SemanchukAnalyzing the Anglo-Saxonicity of the Baby BNC


This is a followup to an earlier post about using Python to measure the “Anglo-Saxonicity” of a text. I’ve used my code to analyze the Baby version of the British National Corpus, and I’ve found some interesting results.

How to Measure Anglo-Saxonicity – With a Ruler or Yardstick?


Thanks to a suggestion from Ben Sizer, I decided to analyze the British National Corpus. I started with the ‘baby’ corpus which, as you might imagine, is smaller than the full corpus.

It’s described as a “100 million word snapshot of British English at the end of the twentieth century“. It categorizes text samples into four groups: academic, conversations, fiction, and news. Below are stack plots showing the percentage of Anglo-Saxon, non-Anglo-Saxon, and unknown words for each document in each of the four groups. The Y axis shows the percentage of words in each category. The numbers along the X axis identify individual documents within the group.

I’ve deliberately given the charts non-specific names of Group A, B, C, and D so that we can play a game. :-)

Before we get to the game, here’s the averages for each group in table form. (The numbers might not add exactly to 100% due to rounding.)

Anglo-Saxon (%) Non-Anglo-Saxon (%) Unknown (%)
Group A 67.0 17.7 15.3
Group B 56.1 25.8 18.1
Group C 72.9 13.2 13.9
Group D 58.6 22.0 19.3

Keep in mind that “unknown” words represent shortcomings in my database more than anything else.

The Game

The Baby BNC is organized into groups of academic, conversations, fiction, and news. Groups A, B, C, and D each represent one of those groups. Which do you think is which?

Click below to reveal the answer to the game and a discussion of the results.


Anglo-Saxon (%) Non-Anglo-Saxon (%) Unknown (%)
A = Fiction 67.0 17.7 15.3
B = Academic 56.1 25.8 18.1
C = Conversations 72.9 13.2 13.9
D = News 58.6 22.0 19.3


With the hubris that only 20/20 hindsight can provide, I’ll say that I don’t find these numbers terribly surprising. Conversations have the highest proportion of Anglo-Saxon (72.9%) and the lowest of non-Anglo-Saxon (13.2%). Conversations are apt to use common words, and the 100 most common words in English are about 95% Anglo-Saxon. The relatively fast pace of conversation doesn’t encourage speakers to pause to search for those uncommon words lest they bore their listener or lose their chance to speak. I think the key here is not the fact that conversations are spoken, but that they’re impromptu. (Impromptu if you’re feeling French, off-the-cuff if you’re more Middle-English-y, or extemporaneous if you want to go full bore Latin.)

Academic writing is on the opposite end of the statistics, with the lowest portion of Anglo-Saxon words (56.1%) and the highest non-Anglo-Saxon (25.8%). Academic writing tends to be more ambitious and precise. Stylistically, it doesn’t shy away from more esoteric words because its audience is, by definition, well-educated. It doesn’t need to stick to the common core of English to get its point across. In addition, those who shaped academia were the educated members of society, and for many centuries education was tied to the church or limited to the gentry, and both spoke a lot of Latin and French. That has probably influenced even the modern day culture of academic writing.

Two of the academic samples managed to use fewer than half Anglo-Saxon words. They are a sample from Colliding Plane Waves in General Relativity (a subject Anglo-Saxons spent little time discussing, I’ll wager) and a sample from The Lancet, the British medical journal (49% and 47% Anglo-Saxon, respectively). It’s worth noting that these samples also displayed highest and 5th highest percentage of words of unknown etymology (26% and 21%, respectively) of the 30 samples in this category. A higher proportion of unknowns depresses the results in the other two categories.

Fiction rests in the middle of this small group of 4 categories, and I’m a little surprised that the percentage of Anglo-Saxon is as high as it is. I feel like fiction lends itself to the kind of description that tends to use more non-Anglo-Saxon words, but in this sample it’s not all that different from conversation.

News stands out for having barely more Anglo-Saxon words than academic writing, and also the highest percentage of words of unknown etymological origin. The news samples are drawn principally from The Independent, The Guardian, The Daily Telegraph, The Belfast Telegraph, The Liverpool Daily Post and Echo, The Northern Echo, and The Scotsman. It would be interesting to analyze each of these groups independently to see if they differ significantly.


My hypothesis that conversations have a high percentage of Anglo-Saxon words because they’re off-the-cuff rather than because they’re spoken is something I can challenge with another experiment. Speeches are also spoken, but they’re often written in advance, without the pressure of immediacy, so the author would have time to reach for a thesaurus. I predict speeches will have an Anglo-Saxon/non-Anglo-Saxon profile closer to that of fiction than of either of the extremes in this data. It might vary dramatically based on speaker and audience, so I’ll have to choose a broad sample to smooth out biases.

I would also like to work with the American National Corpus.

Stay tuned, and let me know in the comments if you have observations or suggestions!





Caktus GroupThe Caktus Success Model

Here at Caktus, we’ve been fortunate to work on some incredible projects with our clients. For example, we built the world’s first SMS voter registration app for Libya, developed a digital archiving system for the world’s largest on-demand video provider, and created a survey tool with auto-scaling architecture to help the University of Chicago track school reform.

While working on these and other projects over the past 10 years, we recognized a set of factors that contribute to development done right, which we call the Caktus Success Model. Those factors are strategic partnerships, a sharp team, focusing on impact, and developing scalable apps. Keep reading for more details on each factor, or download our Shipping Faster white paper for tips on implementing them.

Strategic Partnerships

We look at the word “partnership” a little differently here. Rather than simply focusing on building strong external connections, we actively look for ways to break down internal silos. Our sales and marketing team is invited to sit in on sprint reviews, our technical staff contribute to content for our blog, and all areas of the business are encouraged to communicate openly with each other. As a result, we can offer a more cohesive experience to our clients and deliver applications that take into account the expertise - and expectations - of all of the stakeholders involved.

How does this contribute to our success? It removes barriers to working effectively, improves morale, and helps us actively manage scope for client projects.

Focusing on Impact

We use the Agile Scrum methodology in our work to better prioritize the features that will have the most impact for business value. Because cost and schedule are often less flexible than scope, effective prioritization ensures that the features being worked on are those that are most important to meet project objectives.

We call our approach Most Valuable Features First (MVFF), which is essentially a more approachable way to look at Minimum Viable Product (MVP). We’ve found it does more to encourage buy-in and input from non-technical stakeholders. As part of the MVFF process, stakeholders come to an agreement on what features are the most valuable to them and how high on the priority list they should go. This approach complements our Agile development process well because we receive feedback quickly and can change course if needed.

Scalable Apps

Approaching projects with the assumption that they will eventually grow or change is key to the planning process at Caktus. Maintaining high code quality and building with future scalability in mind is essential to delivering quality web apps. A deadline cannot be allowed to overrule quality work, which is why we insist on 90%+ test coverage, peer code reviews on every pull request, PEP8 coding standards, and following Django best practices.

Although it can seem like this slows things down or limits scope in the short-term, future updates and improvements will come much more easily to projects developed correctly the first time. For our clients, our focus on technical clarity and robustness from the start of the project means that they save time and money on future work. For us, it means we can take pride in our work and spend less time onboarding new developers or new projects.

Building with scalability and future updates in mind is especially important considering that your web app will need regular updates over the course of its lifetime. (Not convinced? We’ve got a blog post sharing 3 big reasons why you should update your Django app.)

A Sharp Team

The sharp team at Caktus is our most valuable asset. Our staff includes core Django contributors, as well as developers dedicated to contributing to the wider open source community. In our hiring process, we look for individuals with depth of vision and a passion for tackling complex challenges. We also strive to build a diverse team, because we find that a variety of perspectives enables us to find solutions that may otherwise be overlooked. Additionally, getting everyone together under one roof and building a co-located team increases the effectiveness of our communication.

It’s not just about what people bring with them when they join Caktus, however. One of our company benefits is a personal development budget for each member of staff and dedicated time off to attend conferences, workshops, and trainings that benefit further career growth. We also hold quarterly ShipIt days to provide dedicated time for our entire team - both developers and non-technical staff - to work on a project or skill of personal interest. Investing in our team once they’re here ensures that the individuals, the company, and our clients all benefit from their growth.

Shipping Faster

Now that you’ve seen the overview of what makes Caktus successful, read more about each factor, including the steps to take to implement each one and more on the benefits of doing so, in our free white paper.

Frank WierzbickiJython 2.7.1 release candidate 3 released!

On behalf of the Jython development team, I'm pleased to announce that the third release candidate of Jython 2.7.1 is available! This is a bugfix release. Bug fixes include improvements in ssl and pip support.

Please see the NEWS file for detailed release notes. This release of Jython requires JDK 7 or above.

This release is being hosted at maven central. There are three main distributions. In order of popularity:
To see all of the files available including checksums, go to the maven query for org.python+Jython and navigate to the appropriate distribution and version.

Caktus GroupHow Documentation Works, and How to Make It Work for Your Project (PyCon 2017 Must-See Talk 3/6)

Part three of six in the 2017 edition of our annual PyCon Must-See Series, highlighting the talks our staff especially loved at PyCon. While there were many great talks, this is our team's shortlist.

A talk that stuck out in my mind from PyCon 2017 was Daniele Procida’s "How documentation works, and how to make it work for your project". In his presentation, he walks through four types of documentation: “Tutorials”, “How-To Guides”, “Reference Guides”, and “Discussions”, giving examples of each, as well as the reasons why each one is valuable to a project.

We value good documentation at Caktus and try to make our projects easily understandable for both technical and non-technical people, so it is always interesting to hear others’ thoughts on the topic. I found Daniele’s presentation clear in terms of why each of the four types of documentation is valuable, and actionable in terms of how to structure documentation within projects.

Caktus GroupTriAgile 2017 Recap

I attended the TriAgile conference in Raleigh, NC on March 30th, 2017 and wanted to share some takeaways from a couple of the talks I saw. Overall, the conference was a nice opportunity to network with some local Agilists and hear about their work and experiences. It was well organized, and I was pleasantly surprised by the size of the Agile community in the Triangle!

The Power of an Agile Mindset

Linda Rising’s keynote talk, "The Power of an Agile Mindset" addressed the differences between fixed versus growth mindsets and how that relates to Agile. She began by outlining an experiment in which students were split into “growth” and “fixed” groups after being given an easy set of questions. Later presented with a series of choices, the fixed group students chose easier tests, felt easily discouraged, showed decreased performance, and lied about their test scores. The growth group students, however, chose more challenging tests, worked hard, enjoyed the challenges, wanted to see tests of students who did better than them, and showed improved performance. (See Mindset by Carol Dweck for further reading on this experiment.)

When boiled down, the fixed mindset views ability as a static attribute that cannot be changed or improved (like height), avoids challenges, is defined by failure, sees effort as being for those who have no talent, and reacts with helplessness in the face of challenge. The growth mindset, or Agile mindset, views ability as an attribute that can be exercised and grown like a muscle, wants to learn, embraces challenge, learns from and is resilient to failure, and recognizes effort as the path to mastery.

When put into the context of an organization, a fixed mindset results in companies who hire only “the best talent” and then continuously fire the lowest performing employees (a practice called “rank and yank”). Growth mindset companies hire based on attitude, focusing on growing their employees, providing learning opportunities, and establishing a culture of trust.

Mindset is important in the practice of Agile at all levels of an organization, from management, to the teams, to the individuals. A fixed mindset can grow to become an Agile mindset through feedback and experimental manipulations (sound familiar?). Rising offered some practical advice for how to reach an Agile mindset, such as praising effort, practice, strategy, and process rather than talent or innate abilities, and encouraging making mistakes and learning from failure rather than punishing or ignoring it.

I found this talk inspiring, and was proud to see that Caktus squarely aligns with the characteristics of the growth mindset companies. Agile is a journey, not a destination: in the end, “reaching the Agile mindset” is not about attaining some nebulous end goal, and more about valuing the process of learning and growing along the way.

The Stances of Coaching

The talk that was most relevant to my role as ScrumMaster was “The Stances of Coaching” by AgileBill Krebs. Krebs’ talk was an introduction to the four “stances” that an Agile Coach must learn to consciously adopt depending on the situation at hand. They are:

  • TEACH: In Teaching stance, the coach relays their expertise on the subject to a group.
  • FACILITATE: In Facilitation stance, the coach leaves their expertise at the door and remains neutral in order to facilitate a group’s conversation/meeting/activity.
  • MENTOR: In Mentoring stance, the coach uses their expertise to help an individual improve.
  • COACH: In Coaching stance, the coach remains neutral and guides an individual on their journey to answer their own questions.

A diagram of the four Agile coaching stances.

The most practical advice that I took away from this talk was to ask powerful questions that are open rather than leading, not stacking questions (asking one at a time and giving the person or group a chance to answer), and getting comfortable with silence. Consciously switching stances can be challenging, but knowing which stance is appropriate will get you halfway there.

Some of the other talks I saw were:

  • “Things to Consider to Ensure a Successful Minimum Viable Product (MVP)” by Thomas Friend, who emphasized dedicating resources to simplifying the MVP in order to focus on core functionality, building and rolling out a small product, and then improving on it once market feedback can be collected.
  • “Why Are There No Women On Our Team? Cognitive Biases We See in Product Development” by Catherine Louis, who explained some common biases and the benefits of having varied and diverse perspectives on development teams, leading to increased creativity, new ideas, and a broader culture.
  • “Consensus Workshop: Group Facilitation to Generate Creative Solutions” by Becca Halstead, who walked the audience through the Consensus Workshop Method in five stages: Context, Brainstorm, Cluster, Name, Resolve. This method helps generate true consensus from a large number of ideas in an engaging way.
  • “Release Control, Become Good Servants” by Camille Spruill, who outlined the characteristics of a good servant-leader, as well as the goals of building teams and communities, empowering others, and bringing out the best in them.

The variety of talks at TriAgile meant there was something on offer for anyone working in an Agile environment. I was tempted to simply follow the Coaching track, but I am glad to have deviated from it in order to see some of the talks that were less directly relevant to my role. I will certainly return for TriAgile 2018, where I also hope to see more pre-conference workshops available.

Caktus GroupPython at Instagram (PyCon 2017 Must-See Talk 2/6)

Part two of six in the 2017 edition of our annual PyCon Must-See Series, highlighting the talks our staff especially loved at PyCon. While there were many great talks, this is our team's shortlist.

One of the talks that I considered a must-see was a keynote presentation by Instagram employees Lisa Guo and Hui Ding about upgrading Python and Django.

This is something that many businesses know they "should" do, but think is too impractical "right now". Instagram performed an upgrade without any downtime and without slowing down the pipeline of new features. Caktus carries out Django upgrades as part of our managed hosting and upgrade protection services, so this talk was especially relevant to what we do as a company.

Tim HopperParallelizing a Python Function for the Extremely Lazy

Do you ever want to be able to run a Python function in parallel on a set of inputs? Have you ever gotten frustrated with the GIL, the multiprocessing library, or joblib?

Try this:

Install Python Fire to run your command from the command line

Install Python Fire with $ pip install fire.

Add this snippet to the bottom of your file:

if __name__ == '__main__':
    import fire

Install GNU Parallel

$ brew install parallel or $ sudo apt-get install parallel may work for you. Otherwise, see this.

Run your function from the command line

$ parallel -j3 "python python_file.py function_name {1} " ::: input1 input2 input3 input4 input5

  • parallel is the command for GNU Parallel.
  • -j3 tells Parallel to run at most 3 processes at once.
  • {1} fills in each item after the ::: as an argument to the function_name.

For example

(lazy) ~ $ cat python_file.py
from time import sleep

def function_name(arg1):
    print("Starting to run with", arg1)
    print("Finishing to run with", arg1)

if __name__ == '__main__':
    import fire
(lazy) ~ $ parallel -j3 --lb  "python -u python_file.py function_name {1} " ::: input1 input2 input3 input4 input5
Starting to run with input2
Starting to run with input1
Starting to run with input3
Finishing to run with input2
Finishing to run with input1
Finishing to run with input3
Starting to run with input4
Starting to run with input5
Finishing to run with input4
Finishing to run with input5

I added --lb and -u to keep Python and Parallel from buffering the output so you can see it being run in parallel.

Tim HopperCondaHTTPError: HTTP 401 UNAUTHORIZED for url

I was getting this message when I tried to install packages from conda-forge with Conda:

Fetching package metadata ...
CondaHTTPError: HTTP 401 UNAUTHORIZED for url <https://conda.anaconda.org/conda-forge/osx-64/repodata.json>
Elapsed: 00:00.920954
CF-RAY: 36ad7cbd5d1c23d8-IAD

The remote server has indicated you are using invalid credentials for this channel.

If the remote site is anaconda.org or follows the Anaconda Server API, you
will need to
  (a) remove the invalid token from your system with `anaconda logout`, optionally
      followed by collecting a new token with `anaconda login`, or
  (b) provide conda with a valid token directly.

Further configuration help can be found at <https://conda.io/docs/config.html>.

I tried to do $ anaconda logout but didn't have a program called anaconda installed.

You can install the Anaconda Cloud Client with $ conda install anaconda-client.

After that, I was able to do $ anaconda logout followed by $ anaconda login where I used my old Binstar credentials (now anaconda.org).

I'm not the only one having this problem.

Caktus GroupDecorators, Unwrapped: How Do They Work? (PyCon 2017 Must-See Talk 1/6)

Part one of six in the 2017 edition of our annual PyCon Must-See Series, highlighting the talks our staff especially loved at PyCon. While there were many great talks, this is our team's shortlist.

Back when I started coding in Python, I remember that decorators were one of the most difficult concepts for me to understand. At the time, I tried to watch a couple of videos about them to better understand them and ended up with no more clarification.

In her talk "Decorators, unwrapped", Katie Silverio uses the example of a “timer” decorator which will log the time any function takes to stdout.

And then, Katie does something really great. She writes some code without any decorators which will act like a decorator. It was a really novel way to teach the audience about how decorators work. I’m looking forward to giving this talk a re-watch the next time I need to use a decorator in some code.

Frank WierzbickiJython 2.7.1 release candidate 2 released!

On behalf of the Jython development team, I'm pleased to announce that the second release candidate of Jython 2.7.1 is available! This is a bugfix release. Bug fixes include improvements in ssl and pip support.

Please see the NEWS file for detailed release notes. This release of Jython requires JDK 7 or above.

This release is being hosted at maven central. There are three main distributions. In order of popularity:
To see all of the files available including checksums, go to the maven query for org.python+Jython and navigate to the appropriate distribution and version.

Caktus GroupSubtests are the Best

Subtests are the best

Testing our code is important. Because developers write bugs, it’s valuable to catch and correct them before the code gets to production so our apps work as they should. Specifically, we want tests that are DRY (Don’t Repeat Yourself), thorough, and readable. Though there are many ways to try to accomplish these goals, subtests make each of them easier. If you’re not using subtests in your test classes, you probably should be.

Subtests were introduced in Python 3.4, and a few uses are briefly covered in the Python documentation. However, there are many more uses that have made my testing better. Their value is probably best seen through an example.

DRYer code and using parameters for cleaner errors

Let’s say we have a function is_user_error() that takes a status code and returns True if it means a user error (note that any 400-level status code is considered user error) has occurred. We could test this function with one test that has many assertions:

import unittest

from ourapp.functions import is_user_error

class IsUserErrorTestCase(unittest.TestCase):
    def test_yes(self):
        """User errors return True."""

But that’s many lines of code, so to be DRYer, we could test the same functionality by writing a for loop:

import unittest

from ourapp.functions import is_user_error

class IsUserErrorTestCase(unittest.TestCase):
    def test_yes(self):
        """User errors return True."""
        for status_code in range(400, 499):

That’s a lot DRYer, and allows us to test many more status codes, but if one of those status codes failed, we would get something like:

FAIL: test_yes (IsUserErrorTestCase)
User errors return True.
Traceback (most recent call last):
  File "test_example.py", line 10, in test_yes
AssertionError: False is not true

Ran 1 test in 0.000s

FAILED (failures=1)

So we’re left wondering, which status code failed? Instead, we can use subtests with parameters:

import unittest

from ourapp.functions import is_user_error

class IsUserErrorTestCase(unittest.TestCase):
    def test_yes(self):
        """User errors return True."""
         for status_code in range(400, 499):
            with self.subTest(status_code=status_code):

Our failure becomes:

FAIL: test_yes (IsUserErrorTestCase) (status_code=405)
User errors return True.
Traceback (most recent call last):
  File "test_example.py", line 10, in test_yes
AssertionError: False is not true

Ran 1 test in 0.002s

FAILED (failures=1)

Which lets us know in the first line that the status_code 405 was the one that failed.

As you can see, subtests give us DRYer code than a single test, and clearer error messages than multiple tests.

DRYer code and using a message (msg) for cleaner errors

Another example: Let's say we're working on a Django project where we created a new API endpoint (/api/people/), and we want to test the fields that are required for it. For the sake of this example, let's say that the endpoint looks like this:

    "id": "",
    "first_name": "",
    "last_name": "",
    "address": "",
    "birthdate": "",
    "favorite_color": "",
    "favorite_number": "",
    "timezone_name": ""

and the required fields for POSTing are: first_name, last_name, and address. One way to test which fields are required would be to write a single test:

from django.test import TestCase

class PeopleEndpointTestCase(TestCase):
    def setUp(self):
        # Do the rest of the setup for logging in the user and giving the user
        # the required permissions

    def test_people_endpoint_post_data(self):
        """Test POSting to the people endpoint with valid and invalid data."""
        url = '/api/people/'
        minimum_required_data = {
            "first_name": "Joe",
            "last_name": "Shmo",
            "address": "123 Fake Street",

        # POSTing with all of the required data
        response = self.client.post(
        self.assertEqual(response.status_code, 201)

        # POSTing with the first_name missing
        data = minimum_required_data.copy()
        response = self.client.post(
         self.assertEqual(response.status_code, 400)

        # POSTing with the last_name missing
        data = minimum_required_data.copy()
        response = self.client.post(
        self.assertEqual(response.status_code, 400)

        # POSTing with the address missing
        data = minimum_required_data.copy()
        response = self.client.post(
        self.assertEqual(response.status_code, 400)

But that's not very DRY, since each section does the same thing. If we split each section into its own test it might be easier to read, but it would be even less DRY. Instead, we could write a loop for each of the sections like this:

from django.test import TestCase

class PeopleEndpointTestCase(TestCase):
    def setUp(self):
        # Do the rest of the setup for logging in the user and giving the user
        # the required permissions

    def test_people_endpoint_post_data(self):
        """Test POSTing to the /people/ endpoint with valid and invalid data."""
        url = '/api/people/'
        minimum_required_data = {
            "first_name": "Joe",
            "last_name": "Shmo",
            "address": "123 Fake Street",

        with self.subTest('POSTing with all of the required data'):
            response = self.client.post(
            self.assertEqual(response.status_code, 201)

        missing_subtests = (
            # A tuple of (field_name, subtest_description)
            ('first_name', 'Missing the first_name field'),
            ('last_name', 'Missing the last_name field'),
            ('address', 'Missing the address field'),
        for field_name, subtest_description in missing_subtests:
            with self.subTest(subtest_description):
                # Remove the missing field from the minimum_required_data
                data = minimum_required_data.copy()

                # POST with the missing field name
                response = self.client.post(
                self.assertEqual(response.status_code, 400)

Now the test uses up fewer lines, and any failures are logged into the console based on the subtest_description.

Independent tests

Another thing to be aware of is that subtests run independently, so we get all of the subtest failures printed into the console, rather than just the first one.

For example, let’s say we’re working on a Django project that has a User model in the our_cool_app app who sometimes creates Blogposts, and we want to track the most recent time that this user has posted, so we create a method called get_latest_activity(). The Blogpost model has a created_datetime field that tracks when it was created, and a creator field that tracks the user that created it.

# models.py
from django.db import models
from django.utils import timezone

from our_cool_app.models import User

class Blogpost(models.Model):
    # ...other fields here...
    created_datetime = models.DateTimeField(default=timezone.now)
    creator = ForeignKey(User, on_delete=models.CASCADE)

We should test that this method works correctly, so we can write the test a few different ways. As one test:

import datetime

from django.test import TestCase
from django.utils import timezone

class UserTestCase(TestCase):
    def test_get_latest_activity(self):
    user = UserFactory()

    # A user with no blogposts has no latest activity

    # A user with 1 blogpost has that blogpost's created_time as the latest activity
    blogpost = BlogpostFactory(creator=user)
    self.assertEqual(user.get_latest_activity(), blogpost.created)

    # A user with multiple blogposts
    a_day_ago = timezone.now() - datetime.timedelta(days=1)
    a_week_ago = timezone.now() - datetime.timedelta(days=7)
    BlogpostFactory(creator=user, created_datetime=a_day_ago)
    BlogpostFactory(creator=user, created_datetime=a_week_ago)
    self.assertEqual(user.get_latest_activity(), blogpost.created)

    # Future blogposts don't count
    tomorrow = timezone.now() + datetime.timedelta(days=1)
    BlogpostFactory(creator=user, created_datetime=tomorrow)
    self.assertEqual(user.get_latest_activity(), blogpost.created)

    # Other people's blogposts don't count
    another_user = UserFactory()
    self.assertEqual(user.get_latest_activity(), blogpost.created)

This looks ok as long as we are careful to make clear comments and use blank lines for better readability. However, if the first assertion (# A user with no blogposts has no latest activity) fails, then the rest of the test doesn’t run. Instead, we can use subtests to write:

import datetime

from django.test import TestCase
from django.utils import timezone

class UserTestCase(TestCase):
    def test_get_latest_activity(self):
        """Test the get_latest_activity() method."""
        user = UserFactory()

        with self.subTest("A user with no blogposts has no latest activity"):

        with self.subTest("A user with one blogpost"):
            blogpost = BlogpostFactory(creator=user)
            self.assertEqual(user.get_latest_activity(), blogpost.created)

        with self.subTest("A user with multiple blogposts"):
            a_day_ago = timezone.now() - datetime.timedelta(days=1)
            a_week_ago = timezone.now() - datetime.timedelta(days=7)
            BlogpostFactory(creator=user, created_datetime=a_day_ago)
            BlogpostFactory(creator=user, created_datetime=a_week_ago)
            self.assertEqual(user.get_latest_activity(), blogpost.created)

        with self.subTest("Future blogposts don't count"):
            tomorrow = timezone.now() + datetime.timedelta(days=1)
            BlogpostFactory(creator=user, created_datetime=tomorrow)
            self.assertEqual(user.get_latest_activity(), blogpost.created)

        with self.subTest("Other people's blogposts don't count"):
            another_user = UserFactory()
            self.assertEqual(user.get_latest_activity(), blogpost.created)

As a result, our code looks more readable, is DRY, and provides useful error messages for each section that fails, rather than just the first one.

A Caveat

While subtests do run independently of each other they don’t run in database transactions, so any changes to the database that are made within one subtest will persist in any subsequent subtests until the end of the test. From our previous example, the user’s first blogpost was created in the “A user with one blogpost” subtest, and it persists until the end of the test (which is why the user’s last activity in the “A user with multiple blogposts” subtest is this first blogpost’s created_datetime).


Subtests allow us to write short DRY sections of code and to have meaningful error messages for when our code fails. There are many ways to use subtests, and more things you can do with them than what is outlined in this blog post, so go and use them to write tests that are thorough, DRY, and readable.

For further reading about newer features of Python, check out New year, New Python: 3.6.

Tim HopperChoice of the Name Dynamic Programming

Richard Bellman quoted by Stuart Dreyfus via Garrett Jones:

I spent the Fall quarter (of 1950) at RAND. My first task was to find a name for multistage decision processes. “An interesting question is, ‘Where did the name, dynamic programming, come from?’ The 1950s were not good years for mathematical research.

We had a very interesting gentleman in Washington named Wilson. He was Secretary of Defense, and he actually had a pathological fear and hatred of the word, research. I’m not using the term lightly; I’m using it precisely. His face would suffuse, he would turn red, and he would get violent if people used the term, research, in his presence. You can imagine how he felt, then, about the term, mathematical.

The RAND Corporation was employed by the Air Force, and the Air Force had Wilson as its boss, essentially. Hence, I felt I had to do something to shield Wilson and the Air Force from the fact that I was really doing mathematics inside the RAND Corporation. What title, what name, could I choose? In the first place I was interested in planning, in decision making, in thinking. But planning, is not a good word for various reasons.

I decided therefore to use the word, ‘programming.’ I wanted to get across the idea that this was dynamic, this was multistage, this was time-varying—I thought, let’s kill two birds with one stone. Let’s take a word that has an absolutely precise meaning, namely dynamic, in the classical physical sense. It also has a very interesting property as an adjective, and that is it’s impossible to use the word, dynamic, in a pejorative sense. Try thinking of some combination that will possibly give it a pejorative meaning. It’s impossible. Thus, I thought dynamic programming was a good name. It was something not even a Congressman could object to. So I used it as an umbrella for my activities.

Caktus GroupPyCon 2017 Recap

Caktus attended PyCon 2017 in Portland from May 18-21. It was the first PyCon for some, while others were PyCon veterans. All of us were looking forward to the opportunity to hear some great talks and make or renew connections in the Python community.

Getting Set Up

Right after arriving from the East Coast on Thursday it was time to set up for Day 1 of the expo. We eagerly prepared for the meet and greet opening night.

Some of the Caktus team at PyCon 2017

Our Ultimate Tic Tac Toe game made a comeback this year with a new AI feature. We only had 2 winners, although one of them beat the AI four times! (Our developers think it’s time to turn the difficulty up to 11.)

Visitors to the Caktus booth playing Ultimate Tic Tac Toe

Meetings and Giveaways

As expected, booth time was busy. It was exciting to welcome so many members of the Python community. We enjoyed chatting about what we do at Caktus and our work building web apps with Django, as well as making connections or meeting with people we haven’t seen in awhile. Mark was excited to catch up with Tom Christie of Django REST Framework.

Technical Director Mark Lavin with Tom Christie of Django REST Framework.

In addition to Caktus swag, we had two prize giveaways on offer this year. The first one was a social contest on Twitter. Christine was our winner by random selection - congratulations!

Winner of the Caktus Twitter contest at PyCon 2017

The second giveaway was a random drawing of people who signed up for our newsletter. Congratulations to Mike for winning that drawing.

Winner of the prize draw at the Caktus booth

Lots to Talk About

Six Caktus developers attended a variety of talks and came out of each of them feeling inspired by fresh ideas.

We can’t wait to see how they apply what they’ve learned to their work at Caktus.

After-hours Event

This year, Caktus hosted an exclusive, invite-only event at Fuse Bar. We were excited to welcome clients and special guests for sushi, nibbles, and refreshments.

Sushi ordered for the Caktus after-hours event.

Some of our clients are based across the country from us, so it was nice to have the opportunity to catch up in person and mingle.

Job Fair

We spoke to lots of interested developers at the PyCon job fair on May 21st. Some of the most common questions we received - and their answers - are below.

Question 1: Where is Caktus based?

Our office is in a lovely brick building in downtown Durham, North Carolina.

Question 2: What does Caktus do / What projects have you worked on?

Caktus is a consultancy and development agency. Work primarily focuses on building custom web and SMS apps in Python, using the Django framework. Some of our projects include building the world’s first SMS-based voter registration system, a live event management app, and responsive, database-driven websites for clients in industries like food and beverage, travel and entertainment.

We also do team augmentation and offer discovery workshops to help our clients strategically plan out the user experience and technical development aspects of their projects. To find out more about our specialties at Caktus, visit our Services page.

Question 3: Do you have opportunities for remote work?

We hire remotely for some contract work, but full-time positions must be based out of our Durham office.

Question 4: Where can I find your current openings / How can I apply?

If you’re interested in joining us at Caktus, check out our Careers page. We’ll need a resume and cover letter, and encourage you to send a link to your GitHub, portfolio, or website as well.

Caktus is growing fast, and we’re pleased to offer great benefits and a fair, equal-opportunity working environment. Why not grow with us?

Until next year!

As always, we had a great time meeting and mingling with our fellow Pythonistas. Thanks to the organizers for putting on another fantastic event, our fellow sponsors for supporting it, the volunteers who kept things moving, and of course, all the attendees for your energy and enthusiasm. See you next year!

Tim HopperBackyard Macro Videography

I've been munching on sunflower seeds while working on my back patio, and some tiny ants (Monomorium minimum, I think) have been enjoying the leftovers.

I pulled out my camera, 50mm lens, and extension tube to experiment with macro videography. The result is quite fun!

Here's what the recording setup looked like.

Caktus Group3 Reasons to Upgrade to the Latest Version of Django

When considering a website upgrade, many business stakeholders probably think about the frontend, i.e., how the website looks or the features users interact with. Perhaps less often considered is the importance of upgrading the backend; that is, the databases, applications, and servers powering all the behind-the-scenes activity. Infrastructure support and upgrades are necessary but often performed as a separate project from any improvements to design or user experience, rather than as part of a holistic update project.

With that in mind, it helps to have an understanding of why upgrading the backend should be considered a necessary part of any website upgrade project. We offer 3 reasons, focusing on our specialty of Django-based websites. Upgrading:

  • increases security,
  • reduces development and maintenance costs, and
  • ensures support for future growth.

Read on for details about each of these factors, or get in touch to speak with us about them.

Increase the security of your site

The Django framework is continually being improved and certain releases are designated as “Long Term Support” (LTS) versions. LTS versions receive security updates and bug fixes for a three-year period, as opposed to the usual 18 months. When your website uses an unsupported version of Django, newly uncovered bugs are not being fixed, patched, or supported by the Django and Open Source communities. No new security fixes are planned for retired versions, a situation that carries a number of risks.

These risks come in the form of vulnerabilities - weaknesses that leave your site open to attack. Attacks could potentially cause servers to go down, data to be leaked or stolen, or features to stop working. If a vulnerability is taken advantage of, it could lead to a loss of reputation and potentially a loss of revenue or legal ramifications. With high consumer expectations and increasing requirements from international data protection laws, this could prove disastrous for organizations or web applications without stringent upgrade plans in place.

If your site is using an older version of Django, a security patch may not be released for your version of Django. This means that a fix for the vulnerability would have to be authored and implemented by your development team, which, over time, is less cost effective than upgrading to the LTS version.

Upgrading to an LTS release offers significant benefits, including security updates as needed. Fixes for security issues and vulnerabilities are implemented quickly. There is no need to implement fixes yourself (or hire out expensive custom work). Taking proactive steps to upgrade reduces risk and can save you the trouble of expensive, reactive steps in the event of a cyberattack.

Reduce development and maintenance costs

In addition to improving security and ensuring support for future growth, upgrading also offers productivity benefits for development teams. Many extra lines of code may be required in order to continue to backport fixes for your website or app as issues occur or features are added. Adding all this code and continuing to use old versions of Django will eventually lead to technical debt, where the short-term fixes and outdated code end up creating extra work to patch and maintain a project in the long run.

Custom fixes and patches also introduce a large learning curve for new developers or contractors. The issue here is two-fold: Onboarding new developers is more time consuming than it needs to be, and if key personnel leave, you may lose knowledge which is integral to maintaining or updating the project.

Upgrading your version of Django reduces technical debt by eliminating old, no-longer-needed code. It also allows your development team to reduce the time and money spent on addressing security issues and bug fixes, freeing up time for them to work on website improvements or revenue-generating work.

Ensure support for future growth

Extensibility is the practice of keeping future growth in mind when working on a development project. We often hear from potential clients who built a website or web app in the early days of their business, when releasing features quickly took precedence over planning for future growth. Whether that growth is in the form of more data, more users, or more functionality, planning for it impacts current design and development decisions. When growth isn’t considered, scaling up the project and adding new features requires a disproportionate amount of work. If the original development was not intended to support the changes being made, custom workarounds must be introduced.

Where does this leave your web project? Technologically out of date, unnecessarily clunky, and less able to deliver a quality experience to site visitors.

Upgrading Django from an out-of-date version to a more recent LTS version not only provides access to software that is constantly receiving bug and security fixes; it also simplifies the upgrade process when a new version of Django is released with a feature needed by your project. If your project is two, three, even four releases behind, upgrading all at once could be cost-prohibitive. By regularly upgrading, you gain near-immediate access to new features in Django if and when needed. In other words, you can depend on a highly-engaged developer community actively working to add features rather than reinventing the wheel by developing them yourself.

Next steps

The wider open source development community is producing great tools and enhancements every day and the community continues to grow in size and support. Your project may find itself left behind if Django is left unsupported - or growing along with the community if you upgrade.

So where to get started? For clients considering an upgrade, we generally advise moving up to the most recent LTS release. While the latest version of Django offers the newest features, the LTS version represents a version of Django that will be more cost efficient to maintain given the community’s three-year commitment to releasing updates for it.

As Django specialists, Caktus developers have experience upgrading and maintaining Django-based websites. We have successfully completed upgrades for numerous clients and offer an upgrade protection plan that ensures your site will be maintained year to year as well as updated to the most recent LTS version of Django. Sound good? Get in touch to start the process of upgrading and securing your website, or take a look at some of our other services if you’ve got a larger project in mind.

Tim HopperAdversarial Learning: Stories of Degradation and Humiliation

My friends Andrew and Joel were kind enough to have me back on their podcast Adversarial Learning. We shared our tales of bad data science interviews. Enjoy!

Caktus GroupUniqueness is an Advantage

Back in March, the organizers from the Women in Tech summit asked if I’d like to collaborate on a panel on diversity in technology at their Philadelphia summit. Back in October, I had created a panel on “Staying a Women in Tech” and was excited for the opportunity to speak on such a significant topic in my hometown of Philadelphia. I was introduced to Brigitte Daniel, who had submitted this new panel, and we began putting the panel plans together. Brigitte would moderate the discussion and I would be a panelist along with three other dynamic women in tech: Elise Wei, Jumoke Dada and Gulrukh Ahanger.

Brigitte, founder of Mogulette, a program centered “around educating, mentoring, and empowering women, with a focus on women of color who are interested in careers in business and technology”, thought it would be a great ice breaker to begin the panel with a clip of Bozoma Saint John at WWDC 2016. Saint John’s ability to connect with her audience on a massive scale set an example for other women of color in the tech industry looking for ways to become more visible and make a name for themselves.

After panel introductions, with panelists ranging from developers and managers to founders and chapter leaders of non-profit organizations that help women learn to code, we began chatting about how we could use our uniqueness to our advantage on our respective teams. Here are some of the highlights captured by our audience on Twitter:

Gulrukh Ahanger

Jumoke Dada

Elise Wei

Erin Mullaney

Overall, I was incredibly happy with how our panel turned out. I definitely heard things from other panelists that I could take back with me and think about. It’s an incredible feeling to be with a group of smart women who are there to help lift each other up.

Gif via Libby VanderPloeg on Giphy

See what other events Cakti have participated in or check out these talks.

Caktus GroupCaktus Consulting Group is an Official AWS Consulting Partner

We’re proud to announce that Caktus has become a certified Amazon Web Services (AWS) Consulting Partner in recognition of the depth and breadth of our AWS expertise. Since AWS became an option for fast, flexible, and low cost infrastructure, we’ve used it to build scalable web or cloud apps for our clients. We’ve used AWS services for computing, networking, storage, databases, security, application services and security for 10 clients over the last few years (and that’s not including the projects we do for fun or as part of ShipIt Day projects).

In addition to our client experience, we have 7 individual AWS certifications amongst our staff. AWS Certification is industry-recognized and demonstrates a thorough, tested knowledge of Amazon Web Services.

We’re looking forward to building more apps with AWS as our top pick for cloud computing services. Joining the Amazon Web Services Partner Network puts us in good company and grants us access to a special range of tools that we can put to work for our clients. To learn more about how we use AWS to deliver highly scalable apps, please contact us.

Or, check out a few of our top blog posts on working with AWS:

Caktus GroupCelebrating 10 Years of Building Web Apps the Right Way

This year marks 10 years of building sharp web apps at Caktus Group. We’re honored by the trust our clients have put in us; it has enabled Caktus to grow from a team of 3 Python developers to an organization of 31 people and supported our efforts to give back to the local and open source communities.

What do Caktus staff have to say about this milestone?

Looking back

Looking back on her 7 years of work at Caktus, Karen says, “It’s been fun to go from 6 people to 30, be a part of that growth and work on communication and defining roles.” She cites her enjoyment of building long-term customer relationships and being able to nurture projects from their early development to completion and improvement over time as her main reason for sticking with Caktus.

Mark, also with Caktus for 7 years, is proud of how his work here and the support of his colleagues provided opportunities to speak at community events and co-author Lightweight Django. He adds, “I feel like I've grown with this company and it's grown with me. I'm proud to say I work here. I remember my first DjangoCon when were were 5-6 people. I told someone I worked at Caktus and they said ‘Oh, I've heard of you guys.’ I knew then that we were doing something right.”

Caktus Top 10s

Top 10 Caktus GitHub contributions

  1. django-project-template
  2. django-scribbler
  3. django-treenav
  4. django-pagelets
  5. fabulaws
  6. django-email-bandit
  7. margarita
  8. django-file-picker
  9. django-jsx
  10. django-comps

Top 10 blog posts in the last year

As part of our commitment to giving back to the development community, we maintain a technical blog with tips for Django, Python, UX, and more. The collection has gotten pretty big over the years! Our 10 most popular blog posts, in order of views, are:

  1. Using Amazon S3 to Store Your Django Site's Static and Media Files
  2. Getting Started Scheduling Tasks with Celery
  3. Migrating to a Custom User Model in Django
  4. Getting Started using Python in Eclipse
  5. Configuring a Jenkins Slave
  6. Custom JOINs with Django's query.join()
  7. Best Python Libraries
  8. Celery in Production
  9. Django Logging Configuration: How the Default Settings Interfere with Yours
  10. Writing Unit Tests for Django Migrations

Building Apps the Right Way

Even with those top 10s, one of the things we’re most proud of is our dedication to building web apps the right way. Caktus CEO Tobias McNulty says, “Doing things right is an ethos that extends to all areas of the business - not just app development. We’ve spent the last 10 years working to implement processes and systems that ensure we continually deliver excellent work to our clients and treat our staff with fairness and respect.”

Caktus' core values guide both our internal and external interactions and have played a key part in growing Caktus to where it is today. We’re confident they will also continue to drive our future growth.

We’ve encountered an immense variety of technical challenges in our 10-year mission to build web applications the right way. It is always a delight to bring that experience to bear on a new project. If you have a project that might benefit from Caktus’ approach, don’t hesitate to get in touch.

Caktus GroupUsing Tokens During Sprint Planning to Allocate Time

In January of 2016, Caktus transitioned from a general Agile development environment to a more focused Scrum environment. Part of this transition entailed moving from a targeted budget allocation approach per project, to a self-organizing, goal-based team structure with no obvious provision for tight, consistent control over project budgets.

If managing budgets is part of your job, you can appreciate how much our project managers struggled with this. We shifted to working in 2-week-long goal-based sprints, but still had to pay attention to budget constraints. We searched for a way to still effectively manage our budgets, but to do so without exercising unseemly amounts of un-Scrum-like “command and control”.

We also noticed that the development team members were having their share of budget-related struggles:

  • If we didn’t discuss hourly budgets in sprint planning, it was difficult for team members to gauge whether the stories they were committing to aligned both with the sprint goals and the project budgets.
  • However, if we did discuss hourly budgets in sprint planning, the teams tended to feel a lack of agency, which inhibited self-organization. This also tended to introduce an unwanted comparison of hours to story points.
  • It was common for the team to commit to stories that appeared to meet the sprint goals, and not realize until the end of the sprint that multiple team members had over-focused on the same project during the sprint. This could lead to overspending on some projects, while underspending on others.
  • The teams realized that without increased transparency regarding budgets, it was entirely possible for them to deliver sprint goals and satisfy client needs, but still come in over- or under-budget.

So how do we maintain our project budgets, empower the team, be truly Agile, and still deliver a working product at the end of each sprint that meets the goals?


Here’s our solution:

  1. Acquire supplies: colored tokens, large pads of paper, markers.
  2. Create a grid on a large piece of paper with enough boxes for your team’s budget sources. These can be projects, sprint activities, time off, etc., but should reflect the main buckets of time that your team members allocate to during sprints. Label each box.
  3. Designate a budget for each token to represent. Our tokens each represent 1 half day (or 4 hours).
  4. During sprint planning, each team member selects a color and takes the amount of tokens equal to their availability during the sprint. Full-time team members at Caktus get 20 tokens; part-time members take fewer as appropriate.
  5. Team members then allocate their tokens in the boxes as they see fit. At this time, the Product Owner (PO) can communicate any budget limitations for specific projects. The team resolves allocation conflicts amongst themselves.

This exercise helps the team identify and resolve these frequent conflicts:

  • Sprint goals not achievable within the budget
  • Over- or under-allocation by individual team members, due to PTO, enthusiasm, or other commitments
  • Project favoritism (everyone wants to work on one project)
  • Project only gets time from a single team member, leaving no space for pull request review or quality assurance from the rest of the team

Typically, our project managers (playing the role of Product Owner) go over the sprint goals prior to this exercise. Once the initial allocation is complete, the team progresses to planning their sprint, starting with the token exercise. This usually takes no more than 5-10 minutes. The grid with the tokens is left in play through the end of the sprint, allowing team members to reference and adjust it as needed. After sprint planning, the Scrum Master posts the initial allocations to the team’s Basecamp.

The budget allocations per person that come out of this exercise are not communicated to the team again during the sprint by the PO, Scrum Master, or other stakeholders. The team can choose to reference the allocations or not, as they see fit. If desired, the PO can compare the initial allocation data to the actual expenditures after the sprint ends. This type of comparison over multiple sprints can be useful in identifying trends that the PO can act upon. For example, if a project is consistently allocated more time in sprint planning than is actually spent on it, yet the goals are always completed, this could be an indication that the goals and/or stories for that project are too small and can be increased in scope.

All three of our Scrum teams are choosing to use this exercise for now. We’ve found that the token exercise provides budget transparency for the development teams, a mechanism for hourly budget management (without command and control) for the POs, and a starting point for team conversations about resource allocation. It also starts sprint planning with a hands-on activity that gets the team thinking and moving around.

Looking for more info about using Scrum and Agile in web development? Read about how we implemented Scrum in a client-services organization, or check out this post about using priority in Scrum to reduce team anxiety.

Caktus GroupShipIt Day Recap Q2 2017

Once per quarter, Caktus employees have the opportunity to take a day away from client work to focus on learning or refreshing skills, testing out ideas, or working on open source contributions. The Q2 2017 ShipIt Day work included building apps, updating open source projects, trying out new tools, and more. Keep reading for the details.

PostgreSQL Performance

Erin used ShipIt Day to watch a tutorial on Postgres performance by Craig Kerstiens and test the Caktus website with some of the things she learned. She used the free pgAdmin III tool to try out some of Craig’s suggested database queries for performance monitoring. While drilling down into our website, she explored cache and index hit rates, reviewed query performance on our blog, and tested with pg_stat_statements to find the most expensive queries in aggregate across database. Erin plans to use her findings to inform decisions impacting website performance.

GitHub Pull Requests Tool

Dan built a tool to help with GitHub pull requests. The tool watches a pull request until it’s ready to merge, then merges it for him. He built it from scratch using the requests library and GitHub API. The tool works by reloading the page occasionally to see if the request is ready to merge, and has tab title changes to make it easy to keep an eye on the status of the request.

Book Club Voting App

Charlotte M built an app to help the Caktus book club vote on their next book. Members can view and add books to the book list for the next election, then vote using an election interface. Once the books are selected, members vote by dragging and dropping the titles in their preferred order to submit votes.

As part of her project, Charlotte researched real-life voting systems and settled on the Borda Count method, preferring a consensus-based system over a majoritarian one.

Open Source Projects

Mark reviewed open source projects and worked on maintenance for the Sick Muse project, a front end for collectd. He wanted to make it work on Python 3 and ensure it works on the latest version of Tornado. While the back end worked, he found that the JavaScript/Bower-based front end broke and plans to remove Bower in future. As part of maintenance, he also worked to improve test coverage from 50% to 88%.

Test Case Management Tool Research

Gerald researched test case management tools that would integrate with JIRA, aiming to find something that would mesh with JIRA as well as sharing a similar visual style. He looked at qTest by QAsymphony, Xray for JIRA, and Zephyr for JIRA, settling on Zephyr for testing. Although it required him to create a few workarounds, Gerald got Zephyr up and running, demonstrating a few user stories and test cases.

For the next ShipIt Day, Gerald plans to look at QA metrics and reporting.

Python for Data Visualization

As part of her grad school projects, NC’s coursework requires at least one semester of Python and data visualization. She spent this ShipIt Day working on creating a media library, working on the exit function and queries for media types which would allow the user to get a list of all of the records that fit a given query.

User Stories for Agile Development

UX designer Basia read User Stories Applied for Agile Software Development by Mike Cohn to brush up her skill in writing user stories as a way to enhance the user story mapping techniques she leverages in discovery workshops. She shared a review of what a user story is and what it conveys, noting that each must be accompanied by acceptance criteria that will validate developed functionality.

She also walked through why user stories should be used in software development, the importance of working as a team to verbally communicate them, the usefulness of user stories in helping to defer details until the team is sure they’re needed, and how they discourage teams from pretending they know up-front everything there is to be known about the project. Most importantly for the Agile developer, she explained how user stories encourage iterative work.

Hello Ansible

Working on an Ansible project for ShipIt Day.

Neil, Dmitriy, and Jeff B worked as a team to start a bare-bones “hello world” project using Ansible and and write an Ansible playbook for its deployment using nginx, gunicorn, Django, Postgres, and memcached.

As a newcomer to devops, Neil liked using Ansible and learned that it’s not as scary as it seems. Dmitriy liked working through the different steps and generally learning about Ansible. Jeff was on board as an advisor, and sees areas where more documentation can be written.

Project Tequila

Vinod worked together with the ‘Hello Ansible’ team on a similar project. Caktus hosts many client projects, all of which were initially created with varying deployment recipes. Many of these use Margarita, Caktus’s homegrown library of Salt recipes. Vinod decided to take one of these older projects (the Libya SMS project) and investigate how it could be migrated from Margarita to Tequila, our internally-developed library for Ansible. This worked surprisingly well (thanks to help from Jeff B, the primary author of Tequila) and by Friday afternoon, we had a single app server deployed successfully to a Vagrant box.


Sarah, Charlotte F, Elizabeth and Gannon created and reviewed posts for the Caktus blog, with topics including sprint planning, conference recaps, and project management. Keep an eye out for those in the next several weeks.

Triangulated Hearts

Kia Lam demonstrates her project for ShipIt Day Q2 2017.

Kia revisited a project built a year and a half ago using the Processing library for animation. The project uses the Triangulate and Minim libraries. The animation of a heart reacts to sound, including voice or a song, by changing color and shifting geometric lines.

For the next version Kia would like to make adjustments to functions built into the library. It’s currently audio reactive but not beat reactive, something she intends to work on.

Or, maybe she’ll animate the Caktus logo for our 10th anniversary party!

Until next time

As you can see, Cakti have been busy on a range of projects. Want to join us and work on sharp web apps? Check out the Caktus careers page for current openings.

Caktus GroupBuilding a Custom Block Template Tag

Building custom tags for Django templates has gotten much easier over the years, with decorators provided that do most of the work when building common, simple kinds of tags.

One area that isn't covered is block tags, the kind of tags that have an opening and ending tag, with content inside that might also need processing by the template engine. (Confusingly, there's a block tag named "block", but I'm talking about block tags in general).

A block tag can do pretty much anything, which is probably why there's not a simple decorator to help write them. In this post, I'm going to walk through building an example block tag that takes arguments that can control its logic.

Django Documentation

There are a couple of pages in the Django documentation that you should at least scan before continuing, and will likely want to consult while reading:

What our example tag will do

Let's write a tag that can make simple changes to its content, changing occurrences of one string to another. We'll call it replace, and usage might look like this:

{% replace old="dog" new="cat" %}
My dog is great.  I love dogs.
{% endreplace %}

which would end up rendered as My cat is great.  I love cats..

We'll also have an optional numeric argument to limit how many times we do the replacement:

{% replace 1 old="dog" new="cat" %}
My dog is great.  I love dogs.
{% endreplace %}

which we'll want to render as My cat is great. I love dogs..

Parsing the template

The first thing we'll write is the compilation function, which Django will call when it's parsing a template and comes across our tag. Conventionally, such functions are called do_<tagname>. We tell Django about our new tag by registering it:

from django import template

register = template.Library()

def do_replace(parser, token):

register.tag('replace', do_replace)

We'll be passed two arguments, parser which is the state of parsing of the template, and token which represents the most recently parsed token in the template - in our case, the contents of our opening template tag. For example, if a template contains {% replace 1 2 foo='bar' %}, then token will contain "replace 1 2 foo='bar'".

To parse that token, I ended up writing the following method as a general-purpose template tag argument parser:

from django.template.base import FilterExpression, kwarg_re

def parse_tag(token, parser):
    Generic template tag parser.

    Returns a three-tuple: (tag_name, args, kwargs)

    tag_name is a string, the name of the tag.

    args is a list of FilterExpressions, from all the arguments that didn't look like kwargs,
    in the order they occurred, including any that were mingled amongst kwargs.

    kwargs is a dictionary mapping kwarg names to FilterExpressions, for all the arguments that
    looked like kwargs, including any that were mingled amongst args.

    (At rendering time, a FilterExpression f can be evaluated by calling f.resolve(context).)
    # Split the tag content into words, respecting quoted strings.
    bits = token.split_contents()

    # Pull out the tag name.
    tag_name = bits.pop(0)

    # Parse the rest of the args, and build FilterExpressions from them so that
    # we can evaluate them later.
    args = []
    kwargs = {}
    for bit in bits:
        # Is this a kwarg or an arg?
        match = kwarg_re.match(bit)
        kwarg_format = match and match.group(1)
        if kwarg_format:
            key, value = match.groups()
            kwargs[key] = FilterExpression(value, parser)
            args.append(FilterExpression(bit, parser))

    return (tag_name, args, kwargs)

Let's work through what that does.

Calling split_contents() on the token is like calling .split(), but it's smart about quoted parameters and will keep them intact. We get back args, a list of strings representing the parts of the template tag invocation, very much like sys.argv gives us for running a program, except that no quotation marks have been stripped away.

The first element in args is our template tag name itself. We remove it because we don't really need it for parsing the args, but save it for generality.

Next we work through the arguments, using the same regular expression as Django's template library to decide which arguments are positional and which are keyword arguments.

The regular expression for keyword arguments also splits on the =, so we can extract the keyword and the value.

We'd like our argument values to support literal values, variables, and even applying filters. We can't actually evaluate our arguments yet, since we're just parsing the template and don't have any particular template context yet where we could look for things like variables. What we do instead is construct a FilterExpression for each one, which parses the syntax of the value, and uses the parser state to find any filters that are referred to.

When all that is done, this method returns a three-tuple: (<tagname>, <args>, <kwargs>).

Our replace tag has two required kwargs and an optional arg. We can check that now:

from django.template import TemplateSyntaxError

# ...

def do_replace(parser, token):
    tag_name, args, kwargs = parse_tag(token, parser)

    usage = '{% {tag_name} [limit] old="fromstring" new="tostring" %} ... {% end{tag_name} %}'.format(tag_name=tag_name)
    if len(args) > 1 or set(kwargs.keys()) != {'old', 'new'}:
        raise TemplateSyntaxError("Usage: %s" % usage)

Note again how we haven't hardcoded the tag name.

Let's pull our limit argument out of the args list:

if args:
    limit = args[0]
    limit = FilterExpression('-1', parser)

If no limit was supplied, we default to -1, which will indicate later that there's no limit. We wrap it in a FilterExpression so we can just call limit.resolve(context) without having to check whether limit is a FilterExpression or not.

We can't check the values here. They might depend on the context, so we'll have to check them at rendering time.

This is all similar to what we might do if we were writing a non-block tag without using any of the helpful decorators that hide some of this detail. But now we need to deal with some unknown amount of template following our opening tag, up to our closing tag. We need to ask the template parser to process everything in the template until we get to our closing tag:

nodelist = parser.parse(('end_replace',))

We get back a NodeList object (django.template.NodeList), which represents a list of template "nodes" representing the parsed part of the template, up to but not including our end tag.

We tell the parser to just ignore our end tag, which is the next token:


Now we're done parsing the part of the template from our opening tag to our closing tag. We have the arguments to our tag in limit and kwargs, and the parsed template between our tags in nodelist.

Django expects our function to return a new node object that stores that information for us to use later when the template is rendered. We haven't written the code for our node object yet, but here's how our parsing function will end:

return ReplaceNode(nodelist, limit=limit, old=kwargs['from'], new=kwargs['to'])

Reviewing what we've done so far

Each time Django comes across {% replace ... %} while parsing a template, it calls do_replace(). We parse all the text from {% replace ... %} to {% endreplace %} and store the result in an instance of ReplaceNode. Later, whenever Django renders the parsed template using a particular context, we'll be able to use that information to render this part of the template.

The node

Let's start coding our template node. All we need it to do so far is to store the information we got from parsing part of the template:

from django import template

class ReplaceNode(template.Node):
    def __init__(self, nodelist, limit, old, new):
        self.nodelist = nodelist
        self.limit = limit
        self.old = old
        self.new = new


As we've seen, the result of parsing a Django template is a NodeList containing a list of node objects. Whenever Django needs to render a template with a particular context, it calls each node object, passing the context, and asks the node object to render itself. It gets back some text from each node, concatenates all the returned pieces of text, and that's the result.

Our node needs its own render method to do this. We can start with a stub:

class ReplaceNode(template.Node):
  def render(self, context):
    return "result"

Now, let's look at those arguments again. We've mentioned that we couldn't validate their values before, because we wouldn't know them until we had a context to evaluate them in.

When we code this, we need to keep in mind Django's policy that in general, render() should fail silently. So we program defensively:

class ReplaceNode(template.Node):
  def render(self, context):
      # Evaluate the arguments in the current context
          limit = int(self.limit.resolve(context))
      except (ValueError, TypeError):
          limit = -1

      from_string = self.old.resolve(context)
      to_string = conditional_escape(self.new.resolve(context))
      # Those should be checked for stringness. Left as an exercise.

Also note that we conditionally escape the replacement string. That might have come from user input, and can't be trusted to be blindly inserted into a web page due to the risk of Cross Site Scripting.

Now we'll render whatever was between our template tags, getting back a string:

content = self.nodelist.render(context)

Finally, do the replacement and return the result:

content = mark_safe(content.replace(from_string, to_string, limit))
return content

We've escaped our own input, and the block contents we got from the template parser should already be escaped too, so we mark the result safe so it won't get double-escaped by accident later.


We've seen, step by step, how to build a custom Django template tag that accepts arguments and works on whole blocks of a template. This example does something pretty simple, but with this foundation, you can create tags that do anything you want with the contents of a block.

If you found this post useful, we have more posts about Django, Python, and many other interesting topics.

Caktus GroupCaktus Activities at PyCon 2017

It’s almost time for PyCon and the team here at Caktus is ready to meet other attendees. Where and how can you find us?

At the booth

Keep an eye out for the Caktus booth in space 232 at the conference expo from May 18-20. Members of our team look forward to welcoming you there, including Colin, David, Julie, and Whitney.

For those interested in learning how Caktus delivers web apps faster, we’ll have copies of our Shipping Faster white paper on hand for review. There will be a prize draw for a Polaroid Cube+ mini action camera, so stop by the booth and let us scan your badge to be entered. The winner will be announced on May 20. Follow Caktus on Twitter for live updates.

Caktus giveaway prizes for PyCon 2017

We’ll also be doing an early giveaway for wireless headphones on May 18 during the opening reception at the expo. Here’s how you can win:

  1. Take a picture of yourself at the Caktus booth
  2. Tweet it to us @CaktusGroup with hashtag #CaktusPyCon

The winner will be randomly selected and announced around 8pm the same evening.

In addition to the prizes we’ll have Caktus swag. Be among the first to get one of our special edition 10th anniversary water bottles!

Caktus swag for PyCon 2017

At the talks and events

Several of our development team are attending as well, so look for them in different talks and events. While some of them are repeat attendees, Dmitriy is looking forward to attending for the first time and listening to talks like Immutable Programming - Writing Functional Python.

Developer Erin is interested in listening to Cython as Secret Weapon for Efficiency. She’s also looking forward to volunteering as a TA at Young Coders: Outside In because of how much she’s enjoyed being a Django Girls coach, and is excited to share her enthusiasm for code with them.

Sarah went to PyCon last year and was very impressed by the inclusiveness of the community. She said, “Everyone is so willing to teach, help, and share with other attendees. I'm looking forward to the testing-centered talks.”

If you’re planning on heading out for the 5k fun run, look out for Mark. He’s also interested in several talks, including Library UX: Using abstraction towards friendlier APIs.

After hours

We’ll be hosting an after hours event on Friday, May 19. Join us for an exclusive happy hour gathering to enjoy some light refreshments. Stop by our booth to get on the invite list.

At the job fair

Caktus is hiring! We’re looking for a Django Web Developer, so stop by table 24 on Sunday, May 21 to talk to members of the team about what it’s like to work with Caktus. We have offices in Durham, NC and Baltimore, MD.

See you soon!

There’s less than a month to go and we can’t wait to meet you. Be sure to contact us to let us know that you’re interested in speaking with us about something in particular, and we’ll be sure to set up a time.

Tim HopperLike most great mathematicians, he expects universal precision

From the Autobiography of Benjamin Franklin:

Thomas Godfrey, a self-taught mathematician, great in his way, and afterward inventor of what is now called Hadley's Quadrant. But he knew little out of his way, and was not a pleasing companion; as, like most great mathematicians I have met with, he expected universal precision in every-thing said, or was for ever denying or distinguishing upon trifles, to the disturbance of all conversation.

I'm a recovering Godfrey Precisionist.

Philip SemanchukThanks, Science!

I took part in the Raleigh March for Science last Saturday. For the opportunity to learn about it, participate in it, photograph it, share it with you — oh, and also for, you know, being alive today — thanks, science!

Caktus GroupCalling all Cat Herders: New Meetup for Digital Project Managers

When I first became a digital project manager (DPM), I struggled to find relevant resources. A ton of information was available on traditional project management, but not much specifically on digital project management. Eventually, I connected with another DPM in my organization and we quickly became friends and confidants. She opened my eyes to the Digital PM Summit, a new conference targeted at DPMs, which was ultimately the inspiration for my new Meetup group.

I attended the Digital PM Summit three times (and even presented last year!). The conference opened up a whole new world to me, one full of practical resources and friendly, helpful contacts. Ever since I first attended the conference, I wanted to replicate some piece of it to connect with DPMs in the Triangle area. It took a few years, but the Triangle Digital Project Managers Meetup has finally come to fruition, thanks in large part to Caktus Group.

The new Meetup is focused on providing opportunities for DPMs in the Research Triangle Area (and beyond) to network, share knowledge, and support each other. No certifications are required to join, and it doesn’t matter what process (or lack thereof) that you use -- Waterfall, Agile, Scrum, or your own Special Secret Sauce. Some of our meetings will be based on a professional topic, while other meetings will be more social. Our goal is to meet at least once every two months.

Project Management is a Team Effort

Over the past few years, I’ve found that many DPMs work solo, or have few cohorts within their organization. At Caktus, we have three full-time project managers and one full-time project management director. PMs at Caktus also serve in the more defined Scrum role of product owner. I’ve been a part of the Caktus team since September 2016, and it’s the first time I’ve been lucky enough to work with a team of DPMs.

When I worked alone, it was often intimidating and even frustrating, because I didn’t have anyone to bounce ideas off of and there was no PM precedent or process already in place. I felt like I was constantly reinventing the wheel. Working in a silo makes connecting with a group of similar professionals even more important, in order to share ideas, stay current, and grow your skills. For these reasons, my team supported my idea to create a Meetup and Caktus, which believes in supporting community involvement, provided me with the time and resources needed to do so.

The Triangle DPM kickoff meeting was held in the Caktus Tech Space at the end of February. The group was small, but passionate and experienced. One attendee even joined remotely via web conference. It was perfect, exactly the kind of inclusive, friendly group that I wanted to bring together. A group where even if you can’t attend in person, you can join remotely and still be home with your kids. A group where everyone is welcome, regardless of industry or job title -- and DPMs have a variety of titles like Digital Producer, Product Owner, or even Account Director! If you manage anything digital, from the Django web development that we do at Caktus to online marketing campaigns or even video games, you’re welcome to join the Meetup.

Come join fellow cat herders at a future meeting. Details will be posted on the Triangle DPM Meetup page and the Caktus events page.

Caktus GroupProduct Discovery Part 2: From User Contexts to Solutions

In the first installment of this two-part series, I introduced product discovery as the process of building a shared understanding about the product between stakeholders and the product team, which helps you make better decisions about what to build. I also suggested that we look at product discovery as a four-step process:

  1. Frame the problem
  2. Identify the users
  3. Map out user actions, tasks, and workflows
  4. Sketch out ideas

Having previously discussed how to frame the problem and identify the users, let’s move on to mapping out user tasks and workflows, and sketching out solutions.

Map out user tasks and workflows

User-centered software design and development arose from the recognition that we must account for human capabilities and characteristics when we build systems and technologies. That’s why so much emphasis is placed on understanding users through research and on empathizing with them by employing tools such as personas and proto-personas.

In order to understand the users, you identify their demographic, psychological, and behavioral characteristics, as well as their goals, needs, pain points, and possible solutions to their challenges. And to place that information in context, you build a narrative within which your users function as they use your product.

User task flowchart

At the very least, when building a product you will create a user flowchart to capture tasks the product should support, decisions the user will be making within the system, user inputs, and system outputs.

While a user flowchart is a useful and succinct way to diagram tasks that need to be supported by an application, there are other methods of capturing user actions that are more story-based and thereby help build a richer representation of user behaviors.

Agile user story mapping

Agile user story mapping is a visualization technique introduced by Jeff Patton that depicts a user’s path through an application. It can also be used to map out user workflows outside of the application.

In Agile software development, a user story is a brief description of a desired feature that is written from the perspective of an end-user, and that captures user outcomes that the feature is meant to support.

Mapping user stories is a group activity in which teams build a narrative about how users engage with software. Using sticky notes, stakeholders and product teams map out user workflows, tasks, task variations, and sub-tasks in chronological order from left to right, and in order of priority or detail from top to bottom.

The resulting user story map is an artifact that offers a quick look at the application’s big picture while preserving a level of detail that can be leveraged to create a backlog. It also shows feature prioritization and can assist in the estimation process.

Example of a user story map created at Caktus for a client. A fragment of a user story map done at Caktus for an animal rescue

User experience mapping

User experience mapping (or user journey mapping) is the process of capturing and communicating complex user interactions across various channels through which the user comes in contact with your product and/or company. It helps build an understanding of user actions, feelings, thoughts, pain and satisfaction points that go beyond the realm of the application itself. The resulting experience (or journey) map provides an omnichannel representation of user experience with touch points at which the user interacts with your product and opportunities to create new or better experiences.

Example of a user journey map created at Caktus for a client. A fragment of a user experience (journey) map created at Caktus for an animal rescue

Narrative arc story mapping

I recently learned about yet another story mapping technique. While I have not had a chance to try it out in my workflow, I found it intriguing and thought it worth sharing.

This approach to story mapping, popularized by Donna Lichaw, relies on a narrative arc as a framework to develop three types of stories — the concept story (the big picture story), the origin story (how your product will be discovered by users), and the usage story (how people use your product). The concept and origin stories are perfect tools for discovering new products, while the usage story can be leveraged to understand the current use and engagement patterns of your product and to identify opportunities for improvements.

Sketch out solutions

By this time in the process, you should know what problem you’re solving and for whom you’re solving it. You should also have a pretty good idea of what workflows and experiences your application should support. Now it’s time to start discussing specific solutions.

Whether in a session involving stakeholders or with your internal team, you can conduct activities that will help you hone in on the right idea. Here are a few suggestions:

  • Ideation: Work individually to produce as many ideas as possible (either by sketching or describing them in writing), then as a team select a small list of solutions that best address the problem at hand.
  • Brainstorming: Work as a team to generate a wealth of ideas by finding inspiration in each other’s concepts, then work together at identifying the best ones.
  • Prototyping: Build paper prototypes, sketch wireframes (on paper or digitally), or code a simple interface to start validating your ideas. You can test your prototypes with team members or recruit a few users and conduct lightweight usability testing.

Get the job done

Each project is unique and product discovery should be tailored to the needs of each project. Whether you develop personas or proto-personas, draw user flowcharts, map user stories, or create an omnichannel user experience map depends on the product you’re building, the resources you have, and the project management paradigm your team follows.

For Agile teams, the lean proto-persona strategy combined with small scale user research and agile story mapping can build a strong foundation of product discovery. But Agile teams will also benefit from the omnichannel perspective of a user journey map that places the product in the context of a broader ecosystem.

At Caktus we often kick off a project with a discovery workshop. The workshop is an opportunity for our team and the client team to get together and build a shared understanding of the product to be built. Working off of existing data or making assumptions where data are lacking, we frame the problem, identify user types, and build personas or proto-personas. In the process, we also identify knowledge gaps and may recommend small scale user research as appropriate. On projects where the problem is well framed and users understood, we work with stakeholders to map out user actions, tasks, and workflows using a technique that best fits the needs and the budget of a given project.

We come out of a workshop with a summary of product goals, identified target user types, and a list of the most valuable content or most valuable product features. If the workshop includes Agile user story mapping, the added benefit is an artifact that can easily be translated into a prioritized backlog. If, during the workshop, we map users’ entire and omnichannel experience, we gain a breadth of understanding of the user journey that goes beyond the application itself, and can support the development of current and future projects.

By establishing in this way a solid, shared understanding of stakeholders’ and users’ needs before any code is committed, we increase our chances of making right decisions about what to build. And by doing so, we reduce the long-term cost because we reduce risks and decrease a need for rework down the road.

Don't forget to read part 1 of this blog post to learn about how to get started with product discovery.

Caktus GroupProduct Discovery Part 1: Getting Started

When setting out to build a new website or web application, it is a good idea to build a shared understanding of the product between stakeholders and the product team. Through research and collaborative activities that aim to answer questions about the product, its goals, and its users’ needs, the stakeholders and product team discover the full breadth and depth of the application to be built, as well as contexts and implications that need to be considered for the product to be successful. We call this process product discovery.

A study conducted by the Institute of Electrical and Electronic Engineers (IEEE) found that software development projects fail when they do not address stakeholders’ needs adequately. It has also been shown that 50% of programmers’ time is spent on avoidable rework. By devoting resources upfront to build a solid, shared understanding of project goals, users, and user contexts, you can ensure that you will be building the right solution and minimizing waste.

Product discovery can be approached as a four-step process:

  1. Frame the problem
  2. Identify the users
  3. Map out user actions, tasks, and workflows
  4. Sketch out ideas

Steps 1 and 2, framing the problem and identifying the users, get you started with understanding the ramifications of your product. Step 3, mapping out user tasks and workflows, is a way to define user contexts and begin exploring solutions. Finally step 4, sketching out ideas, is a step toward articulating a solution.

In this article, I will focus on steps 1 and 2. Steps 3 and 4 will be covered in the second installment of this two-part series.

Frame the problem

When framing the problem(s), you are striving to answer the following questions:

  • What problem(s) am I trying to solve?
  • For whom am I solving this problem?
  • Why am I solving this problem?
  • What does success mean and how can I measure it?
  • What constraints do I need to accommodate?

Answers to these questions may be drawn from your business analytics and existing user or customer research. Data that inform your answers may come from:

  • Competitive audit
  • SWOT (Strengths, Weaknesses, Opportunities, Threats) analysis
  • User or customer interviews or surveys that reveal pain points, needs, and goals
  • Existing use and engagement patterns
  • Points of drop-off or failure, etc.

If data in those areas are lacking, you may start out by making assumptions and stating hypotheses that you will later put to test.

Identify the users

You can identify your users by asking questions such as:

  • What are the demographic, psychological, and behavioral characteristics of the users?
  • What are users’ goals, needs, and pain points?
  • What user outcomes do I need to support?
  • What are the workflows my users employ?
  • How do users interface with my product?
  • How do users leverage technology in their life and/or work?
  • What types of solutions would best serve the users?
  • Other questions about your users’ lives and work, and their interactions with products similar to yours.

You can gain answers to these questions by conducting user research including:

  • Usability testing (observing people using an existing product or a competitor’s product)
  • User interviews (talking to users directly about their workflows, goals, needs, and pain points)
  • User surveys (having users answer questions, usually online)
  • Contextual inquiry (observing users in the context in which they use or would use your product)

Armed with the data, you can then develop user profiles called personas. Personas are tools that allow you to consolidate information about your users into a succinct format, and, perhaps more importantly, give your users a human face. Personas are documented user profiles. But they are also a device that helps you identify with your users and develop empathy for them. It is particularly true in the case of so-called proto-personas — user profiles not based on actual data, but rather on assumptions and guesses you make about your users.

Sample persona created at Caktus for an animal rescue project.

A sample proto-persona done at Caktus for an animal rescue

Personas (or proto-personas) include information grouped into categories, and there are multiple suggestions about good categories to use.

In Lean UX, Eric Ries recommends grouping information about users into:

  • Sketch and name
  • Behavioral and demographic information
  • Pain points and needs
  • Potential solutions

Ladies that UX suggest the following information categories to build a persona:

  • Bio and demographics
  • Emotions and behaviors
  • Goals
  • Solutions

In User Story Mapping, Jeff Patton shares a persona template that includes:

  • User type and role
  • Name and sketch
  • Context
  • About
  • Implications

Next, strive for deeper understanding and explore solutions

Once you’ve gained an understanding of the problem you are solving and the characteristics of the users, you’re ready to dive deeper into user contexts and to start considering solutions. In the next blog post, I will discuss techniques that can be leveraged to explore user contexts and ways to start identifying solutions. Stay tuned!

This blog post continues with Part 2: From User Contexts to Solutions, to be released later this week.

Tim HopperMetawork is more interesting than work

This Software Engineering Radio interview with Neal Ford on Success Skills for Architects is full of gems about building effective software.

He talks a lot about how coders love to solve problems, and that love can lead them to invent interesting, but unnecessary, problems to solve. This is true.

Metawork is more interesting than work. It's so hard to get back to simplicity, because we love complicated little puzzles to solve, so we keep overengineering everything.

Anyone who's developing software would benefit from listening.

Tim HopperTowards Reducing Distractions while Working

Staying focused while working in front of a computer and within reach of a smartphone is hard.

In 2017, teaching people to focus is becoming a industry.

I've been trying to rethink distractions in my own life, particularly in my work environment. Here are some things that have helped:

Working from Home

Working in an office, especially an open-floor plan office, is disastrous for staying focused. DeMarco and Lister wrote about this in Peopleware 30 years ago, and yet open offices are the norm for startups today.

I'm much more productive by working from home in my quiet office or on my back patio. I'm finally able to spend my time thinking about hard problems rather than ways of silencing Constant Throat Clearer or Perpetual Annoying Laugher.


Every app and website these days wants to send you notifications. I'm aggressive about reducing notifications down to those that I need see, and I let almost nothing notify me with sound. I use Do Not Disturb mode on my phone and Mac whenever I need to stop notifications altogether.


Slack has become the new normal for company communication. Some would say Slack itself is ruining our focus, but having it regularly available has been essential for my own work.

I've come up with a few ways to take control of Slack:

  1. Only show "My unread, along with everything I've starred" in the sidebar. See Michael Lopp's excellent post on Slack for more here.
  2. Enable notifications selectively.
  3. Sign out of distracting avocational Slacks.

Social Media

I've started using an app called Focus to block distracting websites (including Facebook and Twitter.com) and apps on my work computer from 9 AM to 5:30 PM. I use Focus's scheduling feature so blocking isn't optional for me.

I've decided not to block Tweetbot. Though it can be distracting, Twitter is an invaluable way for me to learn from my professional colleagues, bounce ideas off of them, and have a good laugh.

On my iPhone, iPad, and personal Laptop, I've started using Freedom to block all social media during the day. This has stopped me from instinctively checking Instagram every time I walk to the bathroom or get suck on a hard problem. I highly recommend it.1

I also use Freedom to block social media for the first hour I'm up in the morning and before I go to bed.


I have two main tactics to keep email from being distracting.

  • I aggressively unsubscribe from mailing lists and ads.
  • I use Sanebox to filter low priority messages out of my inbox.

When emails only need a brief reply, I tend to write responses as soon as possible. At the moment, I'm trying to break people of the expectation that I'll respond quickly. Using services like Boomerang which lets me write emails now and have them sent later helps here.


Long-form reading at the computer is terrible for comprehension. As Doug Lemov has argued, you have to get away from your computer and other devices to read deeply. I do this by printing articles or reading on my iPad with Freedom blocking enabled. I take my printouts or iPad and walk away from my desk to read.

Todo Items

I'm a firm believer in the Getting Things Done principle of reducing the cognitive overhead of tracking to-do items in my head. I use Omnifocus for task management. Mail Drop and this Alfred workflow help me to quickly add tasks to my Omnifocus inbox. When I think of something I need to take care of outside of work, I drop that thought into Omnifocus; this keeps those personal to-do items from distracting me while I'm working.

Staying focused is hard. I'm still learning how to do it well, and I'm sure I'm not the only one struggling to improve here. If you have any tips to share, I'd love to hear them!

  1. I can't use Freedom on my work computer, because it acts as a VPN which conflicts with my work VPN. 

Philip SemanchukHow to Measure Anglo-Saxonicity – With a Ruler or Yardstick?

Summary (Nutshell)

This is a first look at a work in progress. I’m using Python to study text from an etymological perspective. Specifically, I’m measuring how many words in a given English language text have Anglo-Saxon origin. Many people (including myself) think that Anglo-Saxon words convey a different sense than their counterparts of French/Latin origin. To demonstrate the point in a small way, I’ve included a Latin and Anglo-Saxon version of each heading in this blog post.

Background (Milieu)

English is a Germanic language with Scandinavian influence, with a big layer of Old French poured on top. That Old French (Anglo Norman French, to be specific) was principally derived from Latin, so English is a hybrid between two major Indo-European language groups. Those mongrel origins are a big part of why English is messy and rich.

French was introduced to English as the language of conquerers and nobility. French was also the language of some European royalty in the 18th and 19th century, further adding to its reputation as a language associated with high status. Even today, English words with French origins often have higher cultural status than their counterparts with Anglo-Saxon origins (think cuisine versus cooking, illumination versus light, create versus make, and escargot versus snail). By contrast, the Anglo-Saxon words are often considered more visceral (think sea versus ocean, sweat versus perspire, and free versus emancipated — more on that last pair in a moment).

For instance, when taunting someone, you reach for blunt Anglo-Saxon words. “Your mother was a hamster, and your father smelled of elderberries!” is 100% Anglo-Saxon, except for “elderberries” which was coined in Middle English from “elder” and “berry”, both of Anglo-Saxon origin.

A still of the French taunting King Arthur from Monty Python and the Holy GrailWilliam of Normandy in 1067, addressing his English subjects.

Legal documents and government issuances, on the other hand, tend to include more words of Latin/French origin. It’s no coincidence that the Latin/French words “Emancipation Proclamation” describe a legal act, but if you want to stir the heart about emancipation, you say something like “Free at last!”(1)  which is all Anglo-Saxon.

Others have written more eloquently than I about how word origin influences tone (Annalisa Quinn at NPRGemma Varnom, and M. Birch, to suggest a few), so I won’t belabor the point more than I already have. But I wanted to talk about how it inspired the project I’ve been working on.

The Project (The Work)

I should preface this by saying that I Am Not A Linguist, and I don’t even play one on TV.

I thought it would be interesting to perform lexicographical analysis of text from an etymological perspective. My etymological categorization is necessarily simple. When I look at a text, I put each word into one of three etymological categories: Anglo-Saxon, non-Anglo-Saxon, or unknown. From this rough grouping I generate statistics that allow me to compare one text to another.

For instance, does one author consistently use more Anglo-Saxon words than other authors? Does an author’s usage of Anglo-Saxon words change from one work to another? Also of interest to me is the etymology of words as the book progresses from front to back. Do the relative frequencies of etymologies change as the book progresses towards its exciting conclusion? For authors writing in English as a second language, is their word selection influenced by their first language?

All of the questions above can be explored with the tool I’ve written. It’s easier to show the tool’s output than describe it, so here’s an analysis of Lewis Carroll’s 1865 work “Alice’s Adventures in Wonderland”.

The graph below shows the relative frequency of the three etymological categories as the book progresses from beginning to end.

A graphical representation of how the etymological ration of Alice in Wonderland changes as one progresses through the book

This graph shows the relative frequency of the three etymological categories as counting statistics for various part-of-speech categories.

A graphical representation of the counts of words by parts of speech and etymology in Alice in Wonderland

The table below is a more detailed version of the chart immediately above. Some percentages may not add up to 100% due to rounding.

Total %age of All Words Anglo-Saxon non-Anglo-Saxon Unknown
All Words 26624 100% 18233 (68%) 3812 (14%) 4579 (17%)
Unique 3528 13% 1354 (38%) 899 (25%) 1275 (36%)
Nouns 8522 32% 4521 (53%) 2354 (27%) 1647 (19%)
Verbs 5479 20% 2994 (54%) 565 (10%) 1920 (35%)
Adjectives 1639 6% 896 (54%) 375 (22%) 368 (22%)
Adverbs 1974 7% 1348 (68%) 420 (21%) 206 (10%)
Other 9010 33% 8474 (94%) 98 (1%) 438 (4%)

Observations (What I See)

There’s some minor observations to be made here, but the strength of this tool will be in comparative analysis. It’s hard to draw conclusions from one analysis before I have an idea of what’s typical.

For instance, at first glance, the ratio of Anglo-Saxon to non-Anglo-Saxon words looks dramatic, but this says more about English than it does about Carroll. The most common words in English are overwhelmingly Anglo-Saxon in origin. (2)  For the small sample size of works I’ve processed so far (just 8 in total), I can see that it’s common for roughly three quarters of the words to be Anglo-Saxon. Alice in Wonderland isn’t an outlier by that standard.

We can also see that the frequency of Anglo-Saxon words decreases slightly throughout the book. This is the kind of trend that I find interesting, but in this case it’s due to an increase in the number of words of unknown etymology. Sometimes a word’s etymology is truly unknown. More often, though, the etymology is classified as unknown for other reasons. Most likely, it’s simply not in my etymological database (which isn’t very complete yet). Also, the word could be a proper noun, an invented word (like “woodshadows” from James Joyce’s Ulysses), or a word for which the etymology is ambiguous. An example of this last category is “bank” which is Anglo-Saxon in origin when referring to the side of a river, but French/Italian in origin when referring to a place that handles money.

At present, the quantity of words classified as “unknown” is too large for my tastes, and I plan to reduce it by improving both my database and the tool.

Verbs are overrepresented in the “unknown” category. My guess is that this is an artifact of my stemmer having difficulty stemming verbs. (I’m currently using the Snowball Stemmer from NLTK.)

As you can see, at this point it’s easier to draw conclusions about the representation of the data than it is about the data themselves. That leads me to the next (and final) topic in this post.

Future (What’s to Come)

As I said in the introduction, this is an early look at a work in progress. Here’s some of the things I’d like to add –

  • Better etymological data
  • Large scale comparisons of text to look for trends (across authors, genres, etc.)
  • More numeric (rather than visual) descriptions of the data to facilitate automated comparison. One idea is to add the mean and standard deviation of the percentage of Anglo-Saxon words.
  • Open sourcing

If you have any suggestions on how to use this tool or make it more interesting, I’d love to hear them in the comments below. I moderate all comments to filter spam which is yet another Viking influence on England.


Like English itself, “Endnote” is an etymological hybrid. “End” is of Anglo-Saxon origin, while  “note” comes from Old French/Latin.

1. Martin Luther King, Jr. isn’t the only person to have said “Free at last!”, but his use of it is perhaps the most famous. His “I Have a Dream” speech makes brilliant use of etymological contrasts. Many of his memorable phrases in that speech (“I have a dream today”, “Let freedom ring”, “Free at last”) are Anglo-Saxon.

2. In 2014 I pulled from Wikipedia a list of the 100 most common English words. At the time, it contained just four non-Anglo-Saxon words. They were “just” (ME < Latin), “people” (ME < Anglo-French < Latin), “use” (ME < Old French, replaced OE brucan, cognate w/modern Swedish bruk-), and “because” (ME < Fr ‘par cause’). There are lots of ways to count the 100 most common words, and doubtless the list would have been different in Carroll’s day. But my guess is that the presence of Anglo-Saxon hasn’t changed dramatically from that 96% regardless of when and how one counts.

Caktus GroupLearning to ask the right questions, or people

I ask a lot of questions as a developer. Some of them have been more basic, like ‘How do I import a Python function from one file into another?’, and some more complex, like ‘How should we take an API request and return a dynamically-generated PDF as a response?’

As I have continued to learn, a couple things have been particularly beneficial:

  1. Learning to Google the question to find the answer
  2. Finding more advanced developers to answer my questions and guide my thinking

As I have grown as a developer, I have improved at knowing when to use each resource, and each remains an important part of my growth. I’ve gathered some points here that have been helpful to me during my learning, as well as some suggestions on how to help others become sharper developers.

At the beginning:

When learning to develop we oftentimes have direct questions that can be answered by another person, or by searching the internet. I remember having questions such as ‘What is a terminal and a shell?’ or ‘How do I know if something is a Python file?’. An experienced developer can answer these questions quickly, but can also point a beginner in the direction of how to find these answers on the internet. Some important things I learned at this point:

  • The answer to my question is most likely on the internet (Stack Overflow, Stack Exchange, etc.) and I should get better at finding it
  • Asking a more experienced developer may be faster, but figuring out the answer on my own can be more useful for knowing how to find answers successfully in the future
  • It is helpful to have a person help me think through the implications of my questions Thinking critically through my question and what I’m trying to solve is a lot more important than the specific answer

For people new to development, I recommend trying to Google your questions first. It may take some time to figure out how to look things up on Google or Stack Overflow, but these are useful skills that even experienced developers use every day. I also recommend finding an experienced programmer to provide any further clarification and direction - look out for meetups, or, if possible, ask a friend. For some Python meetups around Durham, NC, see TriPython.

For experienced programmers fielding such questions, remember that it’s a lot more important to become better at thinking through issues than to receive a quick answer.

With some experience:

As we gain development experience, we become better able to answer our own questions by doing internet searches, but we also encounter more complex questions, like ‘What happened to cause this timeout error?’ or ‘Is it possible to build an app that does...?’. Again, it’s important to do Google searches to see if other people have asked similar questions, or to ask these questions to more advanced developers. Some things I learned at this point:

  • Other people have probably attempted to use this feature, and either written about it, or documented it
  • There are multiple ways to accomplish what I am trying to do
  • Some features/libraries are better than others
  • It’s always helpful to ask myself, ‘What am I trying to solve here?’
  • More experienced developers oftentimes know how to solve my issue better, so I should ask their opinion

For people asking these questions, I recommend searching the internet not only to find what other people have asked or answered about specific features or libraries but also to ask questions about what a particular feature or library means for the project: What do we want to accomplish with this feature or library? How would this feature affect the user?

The answers to these questions can be beneficial in framing questions about the feature or library to more experienced developers.

With more experience:

Greater experience means that we can usually answer our more basic questions on our own. However, it also leads to different questions, oftentimes changing ‘Can I’ and ‘How does one’ questions into ‘Should I’ and ‘Why would one’ questions. An example may be considering a library and researching its issues, maintenance, and other people’s experience with it, or considering a feature for a project and whether it will lead to our own maintenance issues. Generally, the questions we ask can aid in deciding between the tradeoffs of different options, and this is something that a more experienced developer can help with.

Some things I learned at this point:

  • It is almost always possible to have a feature to do
  • Every feature has tradeoffs
  • I usually haven’t thought of all the tradeoffs or the historical reasons for using certain libraries and not others, but a more experienced developer may have

I encourage people with more complex development questions to come up with some options for solving an issue or adding a feature. Use a Google search or another developer’s experience to research these, and then ask a more experienced developer what the implications of each option might mean for the project.

A caveat:

When searching online, it is always possible to find outdated answers, especially for rapidly-changing topics like JavaScript, or when researching new libraries. While someone’s answer may have worked 5 years ago, the library may have changed and no longer supports the previous method, or the community may have moved along to use different development guidelines or frameworks. To combat this, I recommend limiting Google searches to just the past year or two, or try to see if older answers have newer comments with more accurate information. Another possibility is finding options on the internet and asking a more experienced developer which one is a better solution.

Further thoughts:

As I have continued learning as a developer, I have become better at knowing what to search for on the internet, and also at asking questions of more experienced developers. I still have lots of basic questions, and now also some more complex ones. While practice makes us better at searching for answers on the internet, I am particularly grateful to be able to ask questions of more experienced developers and to allow them to guide my thinking about certain topics. In fact, I have really relied on the expertise of more experienced developers to guide the way that I approach technical issues and figure out the best way to resolve them. I encourage any tech firm to support these kinds of interactions between developers, whether in a formal way (mentorship meetings, hosting meetups, etc.) or informally. I know it has made my time at Caktus both more enjoyable and more efficient.

Becoming a better developer is an ongoing process. Check out how to plan for mistakes as a developer to continue the journey.

Caktus GroupDigging Into Django QuerySets

Digging Into Django QuerySets

Object-relational mappers (or ORMs for short), such as the one that comes built-in with Django, make it easy for even new developers to become productive without needing to have a large body of knowledge about how to make use of relational databases. They abstract away the details of database access, replacing tables with declarative model classes and queries with chains of method calls. Since this is all done in standard Python developers can build on top of it further, adding instance methods to a model to wrap reusable pieces of logic. However, the abstraction provided by ORMs is not perfect. There are pitfalls lurking for unwary developers, such as the N + 1 problem. On the bright side, it is not difficult to explore and gain a better understanding of Django's ORM. Taking the time and effort to do so will help you become a better Django developer.

In this article I'll be setting up a simple example app, consisting of nothing more than a few models, and then making use of the Django shell to perform various queries and examine the results. You don't have to follow along, but it is recommended that you do so.

First, create a clean virtualenv. Here I'll be using Python 3 all of the way, but there should be little difference with Python 2.

$ mkvirtualenv -p $(which python3) querysets
Already using interpreter /usr/bin/python3
Using base prefix '/usr'
New python executable in /home/jrb/.virtualenvs/querysets/bin/python3
Also creating executable in /home/jrb/.virtualenvs/querysets/bin/python
Installing setuptools, pkg_resources, pip, wheel...done.

Next, install Django and IPython,

(querysets) $ pip install django ipython

Create the new project.

(querysets) $ django-admin.py startproject querysets
(querysets) $ cd querysets/
(querysets) $ ./manage.py startapp qs

Update querysets/settings.py to add 'qs', to the end of the INSTALLED_APPS list. Then, edit qs/models.py to add the simple models we will be dealing with

from django.db import models

class OnlyOne(models.Model):
    name = models.CharField(max_length=16)

class MainModel(models.Model):
    name = models.CharField(max_length=16)
    one = models.ForeignKey(OnlyOne)

class RelatedModel(models.Model):
    name = models.CharField(max_length=16)
    main = models.ForeignKey(MainModel, related_name='many')

Finally, set up the database.

(querysets) jrb@caktus025:~/caktus/querysets$ ./manage.py makemigrations qs
Migrations for 'qs':
    - Create model MainModel
    - Create model OnlyOne
    - Create model RelatedModel
    - Add field one to mainmodel
(querysets) jrb@caktus025:~/caktus/querysets$ ./manage.py migrate

Running python manage.py shell should now pull up an IPython session.

Now that we have a working project set up, we'll need some means of keeping track of the quantity and the raw SQL of any queries sent to the database. Django's TransactionTestCase class provides an assertNumQueries method, which is interesting but too specific and too tied to the test suite for our needs. However, examining its implementation, we can see that it ultimately makes use of a context manager called CaptureQueriesContext, from the django.test.utils module. This context manager will cause a database connection to capture all of the SQL queries sent, even if such is currently turned off (i.e. if DEBUG = False is set), and make those queries available on the context object. I find this a useful tool to use in debugging to track down code that is issuing too many queries to the database, in situations where Django Debug Toolbar won't help.

At the time of writing, the most recent released version of Django is 1.10.6. I've copied the code for CaptureQueriesContext for this version below, with a few irrelevancies redacted.

class CaptureQueriesContext(object):
    def __init__(self, connection):
        self.connection = connection

    def captured_queries(self):
        return self.connection.queries[self.initial_queries:self.final_queries]

    def __enter__(self):
        self.force_debug_cursor = self.connection.force_debug_cursor
        self.connection.force_debug_cursor = True
        self.initial_queries = len(self.connection.queries_log)
        self.final_queries = None
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        self.connection.force_debug_cursor = self.force_debug_cursor
        if exc_type is not None:
        self.final_queries = len(self.connection.queries_log)

So here we can see several things of interest to us. The context manager keeps a reference to the database connection (as self.connection), it sets and then unsets a flag on the connection (self.connection.force_debug_cursor) which tells the connection to do the captures, it stores the number of queries at the start and at the end (self.initial_queries and self.final_queries), and finally, it provides a slice of the actual queries captured as the property captured_queries. Nothing here restricts its use to the test suite, so we'll be making use of it throughout in our IPython session.

Let's try it out now.

In [1]: from django.test.utils import CaptureQueriesContext

In [2]: from django.db import connection

In [3]: from qs import models

In [4]: with CaptureQueriesContext(connection) as context:
   ...:     print(models.MainModel.objects.all())
<QuerySet []>

In [5]: print(context.initial_queries, context.final_queries)
0 1

So we can see that there were no queries to start out with, and that a query was issued to the database by our code. Let's see what that looks like,

In [6]: print(context.captured_queries)
[{'time': '0.001', 'sql': 'SELECT "qs_mainmodel"."id", "qs_mainmodel"."name", "q
s_mainmodel"."one_id" FROM "qs_mainmodel" LIMIT 21'}]

This shows us that the captured_queries property gives us a list of dicts, and each dict contains the raw SQL and the time it took to execute. In the above query, note the LIMIT 21. This is there because the repr() of a QuerySet limits itself to showing no more than 20 of the items it contains. The additional twenty-first item is captured so that it knows whether or not to add an ellipsis at the end to indicate that there are more items available.

Let's create some data. First up, we need a quick and dirty way of populating the name fields

In [7]: import random

In [8]: import string

In [9]: def random_name():
   ...:     return ''.join(random.choice(string.ascii_letters) for i in range(16
   ...: ))

In [10]: random_name()
Out[10]: 'nRtybzKaSZWjHOBZ'

Now the objects

In [11]: with CaptureQueriesContext(connection) as context:
    ...:     models.OnlyOne.objects.bulk_create([
    ...:         models.OnlyOne(name=random_name())
    ...:         for i in range(5)
    ...:     ])
    ...:     models.MainModel.objects.bulk_create([
    ...:         models.MainModel(name=random_name(), one_id=i + 1)
    ...:         for i in range(5)
    ...:     ])
    ...:     models.RelatedModel.objects.bulk_create([
    ...:         models.RelatedModel(name=random_name(), main_id=i + 1)
    ...:         for i in range(5)
    ...:         for x in range(7)
    ...:     ])

In [12]: print(context.final_queries - context.initial_queries)

In [13]: print(context.captured_queries)
[{'sql': 'BEGIN', 'time': '0.000'}, {'sql': 'INSERT INTO "qs_onlyone" ("name") S
GTwPkXTUSpZYBWCT\'', 'time': '0.000'}, {'sql': 'BEGIN', 'time': '0.000'}, {'sql'
: 'INSERT INTO "qs_mainmodel" ("name", "one_id") SELECT \'fsekHOfSJxdiGiqp\', 1
, 5', 'time': '0.000'}, {'sql': 'BEGIN', 'time': '0.000'}, {'sql': 'INSERT INTO
"qs_relatedmodel" ("name", "main_id") SELECT \'tMOCzPRjKZHbwBLb\', 1 UNION ALL S
'BKgXGwdXJQBMQGJM\', 5', 'time': '0.000'}]

This looks pretty ugly, but we can see that each .bulk_create() results in two queries, a BEGIN starting the transaction, and an INSERT INTO with a crazy set of SELECT and UNION ALL clauses following it.

Ok, now that we are finally all set up, let's explore. What happens if we just create a QuerySet and set it in a variable?

In [14]: with CaptureQueriesContext(connection) as context:
    ...:     qs = models.MainModel.objects.all()

In [15]: print(context.final_queries - context.initial_queries)

In [16]: print(context.captured_queries)

No queries were sent to the database! This is because a Django QuerySet is a lazy object. It contains all of the information it needs to populate itself from the database, but will not actually do so until the information is needed. Similarly, .filter(), .exclude(), and the other QuerySet-returning methods will not, by themselves, trigger a query sent to the database.

In [17]: with CaptureQueriesContext(connection) as context:
    ...:     qs = models.MainModel.objects.filter(name='foo')
    ...: print(context.final_queries - context.initial_queries)

In [18]: with CaptureQueriesContext(connection) as context:
    ...:     qs = models.MainModel.objects.filter(name='foo')
    ...:     qs2 = qs.filter(name='bar')
    ...: print(context.final_queries - context.initial_queries)

Here we see that even chaining a filtered QuerySet off of another QuerySet is insufficient to cause a database access. However, non-QuerySet-returning methods such as .count() will result in a query sent to the database.

In [19]: with CaptureQueriesContext(connection) as context:
    ...:     count = models.MainModel.objects.count()
    ...: print(context.final_queries - context.initial_queries)

So, when will a QuerySet result in a round-trip to the database? Basically, this happens any time concrete results are needed from the QuerySet, such as looping explicitly or implicitly. Here are some of the more typical ones

In [20]: with CaptureQueriesContext(connection) as context:
    ...:     for m in models.MainModel.objects.all():
    ...:         obj = m
    ...:     r = repr(models.OnlyOne.objects.all())
    ...:     l = len(models.RelatedModel.objects.all())
    ...:     list_main = list(models.MainModel.objects.all())
    ...:     b = bool(models.OnlyOne.objects.all())
    ...: print(context.final_queries - context.initial_queries)

Note that each of these triggers its own query. The Django docs have a full list of the things that cause a QuerySet to trigger a query.

We've now seen that simply instantiating a QuerySet doesn't send a query to the database, and that obtaining data out of it does. The next most obvious question is, will a QuerySet ask for data from the database multiple times? Let's find out

In [21]: with CaptureQueriesContext(connection) as context:
    ...:     qs = models.MainModel.objects.all()
    ...:     L = list(qs)
    ...:     L2 = list(qs)
    ...: print(context.final_queries - context.initial_queries)

Terrific! Just as we would hope, the QuerySet somehow reuses its previous data when we ask for it again. Keep in mind, though, if we attempt to further refine a QuerySet,

In [22]: with CaptureQueriesContext(connection) as context:
    ...:     qs = models.MainModel.objects.all()
    ...:     L = list(qs)
    ...:     qs2 = qs.filter(name__startswith='b')
    ...:     L2 = list(qs2)
    ...: print(context.final_queries - context.initial_queries)

it does not re-use the data. So how does this work? The implementation of QuerySet can be found in django.db.models.query, but in particular, let's look at the implementation of the relevant methods

def __iter__(self):
    return iter(self._result_cache)

def _fetch_all(self):
    if self._result_cache is None:
        self._result_cache = list(self.iterator())
    if self._prefetch_related_lookups and not self._prefetch_done:

def iterator(self):
    return iter(self._iterable_class(self))

So we can see that iterating over a QuerySet will check to see if a cache at ._result_cache is populated yet, and if not, populates it with a list of objects. This list, then, is what will be iterated over. Subsequent iterations will then get the cache, so no further queries are issued. Doing a chained .filter() call, though, results in a new QuerySet that does not share the cache of the previous one.

The iterator() method used above is a documented public method, which returns an iterator over a configurable iterable class of model instances. Note that it does not involve the cache, so subsequent calls will result in a new query to the database. So why is this a public method? Under what circumstances would it be useful to not populate the cache? The iterator() method is most useful when you have memory concerns when iterating over a particularly large QuerySet, or one that has a large amount of data stored in the fields, especially if it is known that the QuerySet will only be used once and then thrown away.

Interestingly, certain non-QuerySet-returning methods such as .count(),

In [23]: with CaptureQueriesContext(connection) as context:
    ...:     qs = models.MainModel.objects.all()
    ...:     L = list(qs)
    ...:     c = qs.count()
    ...: print(context.final_queries - context.initial_queries)

can also make use of an already filled cache.

A common pattern that you will see is iterating over a QuerySet in a template, and rendering information about each item, which may involve access of related objects. To simulate this, let's loop and set the name of each item's OnlyOne into a variable.

In [24]: with CaptureQueriesContext(connection) as context:
    ...:     for item in models.MainModel.objects.all():
    ...:         name = item.one.name
    ...: print(context.final_queries - context.initial_queries)

Six queries! What could possibly be going on here?

In [25]: for q in context.captured_queries:
    ...:     print(q['sql'])
SELECT "qs_mainmodel"."id", "qs_mainmodel"."name", "qs_mainmodel"."one_id" FROM "qs_mainmodel"
SELECT "qs_onlyone"."id", "qs_onlyone"."name" FROM "qs_onlyone" WHERE "qs_onlyone"."id" = 1
SELECT "qs_onlyone"."id", "qs_onlyone"."name" FROM "qs_onlyone" WHERE "qs_onlyone"."id" = 2
SELECT "qs_onlyone"."id", "qs_onlyone"."name" FROM "qs_onlyone" WHERE "qs_onlyone"."id" = 3
SELECT "qs_onlyone"."id", "qs_onlyone"."name" FROM "qs_onlyone" WHERE "qs_onlyone"."id" = 4
SELECT "qs_onlyone"."id", "qs_onlyone"."name" FROM "qs_onlyone" WHERE "qs_onlyone"."id" = 5

As we can see, we have one query which populates the main QuerySet, but then as each item gets processed, each sends an additional query to get the item's associated OnlyOne object. This is referred to as the N + 1 Problem. But how can we fix it? It turns out that Django comes with a QuerySet method for just this purpose: select_related(). If we adjust our code like this,

In [26]: with CaptureQueriesContext(connection) as context:
    ...:     for item in models.MainModel.objects.select_related('one').all():
    ...:         name = item.one.name
    ...: print(context.final_queries - context.initial_queries)

we drop back down to only one query again

In [27]: for q in context.captured_queries:
    ...:     print(q['sql'])
SELECT "qs_mainmodel"."id", "qs_mainmodel"."name", "qs_mainmodel"."one_id", "qs_
onlyone"."id", "qs_onlyone"."name" FROM "qs_mainmodel" INNER JOIN "qs_onlyone" O
N ("qs_mainmodel"."one_id" = "qs_onlyone"."id")

So .select_related('one') tells Django to do an INNER JOIN across the foreign key, and make use of that information when instantiating the objects in Python. Great! The select_related() method is capable of taking multiple arguments and will do a join for each of them. You can also join multiple tables deep by using Django's double-underscore syntax, for example .select_related('foo__bar') would join our main model's table with the table for 'foo', and then further join with the table for 'bar'. Note that other things that would cause a join in the sql, such as filtering on a field on the related object, will not cause that related object to be made available as a Python object; you still need to specify your .select_related() fields explicitly.

This all works if the model we are querying has a foreign key to the other model. What if the relationship runs the other way, resulting in a one-to-many relationship?

In [29]: with CaptureQueriesContext(connection) as context:
    ...:     for item in models.MainModel.objects.all():
    ...:         for related in item.many.all():
    ...:             name = related.name
    ...: print(context.final_queries - context.initial_queries)

In [30]: for q in context.captured_queries:
    ...:     print(q['sql'])
SELECT "qs_mainmodel"."id", "qs_mainmodel"."name", "qs_mainmodel"."one_id" FROM
SELECT "qs_relatedmodel"."id", "qs_relatedmodel"."name", "qs_relatedmodel"."main
_id" FROM "qs_relatedmodel" WHERE "qs_relatedmodel"."main_id" = 1
SELECT "qs_relatedmodel"."id", "qs_relatedmodel"."name", "qs_relatedmodel"."main
_id" FROM "qs_relatedmodel" WHERE "qs_relatedmodel"."main_id" = 2
SELECT "qs_relatedmodel"."id", "qs_relatedmodel"."name", "qs_relatedmodel"."main
_id" FROM "qs_relatedmodel" WHERE "qs_relatedmodel"."main_id" = 3
SELECT "qs_relatedmodel"."id", "qs_relatedmodel"."name", "qs_relatedmodel"."main
_id" FROM "qs_relatedmodel" WHERE "qs_relatedmodel"."main_id" = 4
SELECT "qs_relatedmodel"."id", "qs_relatedmodel"."name", "qs_relatedmodel"."main
_id" FROM "qs_relatedmodel" WHERE "qs_relatedmodel"."main_id" = 5

As before, we get 6 queries. However, if we were to try to use .select_related('many'), we would get a FieldError. For this situation, Django provides a different method to mitigate the problem: prefetch_related.

In [31]: with CaptureQueriesContext(connection) as context:
    ...:     for item in models.MainModel.objects.prefetch_related('many').all()
    ...: :
    ...:         for related in item.many.all():
    ...:             name = related.name
    ...: print(context.final_queries - context.initial_queries)

Two queries, that's at least better. What's going on here, though, why two? If we take a look at the queries generated, we see

In [32]: for q in context.captured_queries:
    ...:     print(q['sql'])
SELECT "qs_mainmodel"."id", "qs_mainmodel"."name", "qs_mainmodel"."one_id" FROM
SELECT "qs_relatedmodel"."id", "qs_relatedmodel"."name", "qs_relatedmodel"."main
_id" FROM "qs_relatedmodel" WHERE "qs_relatedmodel"."main_id" IN (1, 2, 3, 4, 5)

So it turns out that Django first loads up the QuerySet for MainModel, then it determines what primary key values it received, and then does a second query on RelatedModel, filtering on those that have a foreign key to one of those values.

There is one thing that you should be aware of when prefetching one-to-many relationships in this manner. A fairly typical thing to do is to make use of Django model's object-oriented nature, and write instance methods that do some non-trivial computation, sometimes involving looping or filtering on one-to-many or many-to-many relationships. We'll simulate that here by just using a .filter() call in the inner loop

In [33]: with CaptureQueriesContext(connection) as context:
    ...:     for item in models.MainModel.objects.prefetch_related('many').all()
    ...: :
    ...:         for related in item.many.filter(name__startswith='b'):
    ...:             name = related.name
    ...: print(context.final_queries - context.initial_queries)

And now we find that we're back up to seven queries, despite the use of .prefetch_related(). What's going on here is that the prefetch is making item.many.all() act exactly like an already iterated-over QuerySet, like from earlier in this article, by filling its cache for later re-use. However, as in those earlier cases, if you do any further refinement of the QuerySet it does not share the cache with the new QuerySet. In many cases, it would simply be better to iterate over the relationship and filter using Python directly. Additionally, Django starting with version 1.7 introduced a Prefetch object, which allows more control over the query used in the prefetch_related() call. I advise using tools such as Django Debug Toolbar, using real data, to determine what makes the most sense for your use.

There is another thing that you should be aware of when encapsulating queries involving one-to-many or many-to-many relationships. You may see code like this

def some_expensive_calculation(self):
    related_objs = RelatedModel.objects.filter(main=self)

This code, as we should now be able to see, is an anti-pattern that will always issue a query when called from a MainModel item, regardless of whatever optimizations have been used on the QuerySet which obtained the MainModel in the first place. It would be better to do this instead

def some_expensive_calculation(self):
    related_objs = self.many.all()

That way, if we have calling code that does this

for item in models.MainModel.objects.prefetch_related('many'):
    result = item.some_expensive_calculation()

we should only get the two queries we expect, not one for the main set plus one each for however many items are in that set.

So now we've seen that the QuerySets that you use in your apps can have significant real-world performance implications. However, with some care and understanding of the simple concepts behind Django's QuerySets, you can improve your code and become a better Django developer. But more than that, I hope that you take away from this article the realization that you shouldn't be afraid to read Django's source code to see how something works, or to build minimal working examples or simple tools to explore problems within the Django shell.

Read more Django posts on the Caktus blog.

Tim HopperWeb Development and Design for the Backend Developer

I've been tinkering with websites for nearly 20 years. My friend Hunter and I were big into making terrible Angelfire sites as pre-teens. In high school, my dad paid me to make him a webpage for his doctor's office (I used Frontpage). A year or two after that, I read Kevin Yank's "Build Your Own Database Driven Website Using PHP & MySQL" and hacked together a PHP back-end for a Lord of the Rings fan site.

In recent years, I've put together this blog, shouldigetaphd.com, and a few other simple web-based side projects. However, I haven't kept up with modern web development, and my projects have been hacked together from boilerplate or templates. I've programmed professionally since 2011, I've spent very little of that writing anything close to graphical user interfaces.

I have a number of other side projects that I'd like to do at some point, and most of them would require some sort of graphical interface. While I could work on app development, I think web-based implementations would be a great starting place.

A few months back, I decided to stop watching Netflix on the treadmill and instead use those 45 minutes each morning to learn; in particular, I've been trying to learn more about modern(ish) web design and development. My work has a subscription to Safari Books Online which gives me access to copious technical books and video tutorials.

The number of resources available on Safari (along with YouTube, blog posts, etc) is astounding. I started many video tutorials on Safari that I quickly realized weren't going to be useful. Yet there many gems to be found, which I share here with you.

What follows is an overview of the technologies I've realized I need to learn more about and links to the resources I've found valuable in learning about them. If you think there are gaps I haven't yet filled or better resources than I've listed below, I'd love your feedback.

What I Knew Going In

I've been a professional software developer and data scientist since 2012. I mostly write Python, but I've programmed in a number of different languages.

I have a pretty good grasp on how HTML and CSS work. I've used enough Javascript over the years to be dangerous; I understood how it runs in the browsers. I understand what a DOM is and how it relates to the page source.

I've used the Python Flask web framework for several projects. I understand how to repond to HTTP requests with server-generated content. I had some idea of how to run my own web server on AWS.

I've used Jekyll, Hugo, and Pelican to create statically generated sites.

I understood DNS at a high level, but never really learned what all the different DNS types were, and I didn't understand why name server changes take so long to propagate.

I had some idea of what node.js and npm are.

I'm a committed Sublime Text user.

A Meta Tutorial on Web Development

A great place to start is Andrew Montalenti's lengthy tutorial on using Python, Flask, Bootstrap, and Mongo to rapidly prototype a website. The tutorial is out of date, but the principles still stand.

Another great resource is Cody Lindley's free Front-End Developer's Handbook. This is a substantial list meta-resource that organizes links for learning all angles of front-end development. "It is specifically written with the intention of being a professional resource for potential and currently practicing front-end developers to equip themselves with learning materials and development tools."

Chrome Developer Tools

One of the most important tools for me in learning more about web development has been the Chrome Developer Tools. You can live edit the DOM elements and style sheets and watch how a website changes. I've mostly learned Developer Tools through exploring it myself, but there are lots of tutorials for it on Youtube.

HTML, CSS, and Bootstrap

Many modern websites are responsive: they automatically adapt to various size screens and devices, from phones to desktops. Writing responsive websites from scratch requires deep knowledge of HTML, CSS, Javascript, and browsers. Unless you're doing this professionally, you probably don't want to write a responsive site from scratch.

For several projects, I've used the lightweight Skeleton project to create simple, responsive pages.

Recently, I decide to dive deep into the more robust Bootstrap framework originally developed at Twitter.

I watched Brock Nunn's Building a Responsive Website with Bootstrap (Safari), a two hour tutorial on getting started with Bootstrap. The documentation for Bootstrap is clear (if terse) and worth reading through.

Once you have a basic idea of how Bootstrap works, the best thing you can do is start playing with it. Since I was familiar with the Pelican static site generator, I decided to switch this blog to Bootstrap theme starting with pelican-bootstrap3.

I've worked with Bootstrap 3 until now. Bootstrap 4 is about to come out. Bootstrap 4 moves the style sheets from LESS to SASS and adds Flexbox functionality. Unless you understand what those mean (more below), you'd be fine using version 3.

I wanted to get a better grasp on CSS Selectors, so I read Eric Meyer's brief Selectors, Specificity, and the Cascade: Applying CSS3 to Documents (Safari)

I watched Marty Hall's JavaScript, jQuery, and jQuery UI tutorial (Safari). I was able to skip big chunks where I already understood certain parts, but it helped me fill in lots of gaps.

Advanced Stylesheets (LESS, SASS, and Flexbox)

There are several alternatives to writing raw CSS. Two popular ones are Less and SASS. These "preprocessors" allow you to write CSS-like stylesheets but with constructs such as variables, nesting, inheritance, and mathematical operators.

I found this brief tutorial on Less (Safari) helpful, and I've enjoyed Less a lot. I haven't used SASS yet, but it's very similar. I'll probably switch to SASS when I start using Bootstrap 4.

Another modern innovation is the Flexbox layout model for CSS. Stone River Learning has a great tutorial on Flexbox (Safari). It seems that Flexbox is the future of CSS-based layouts, and it's worth learning about.

Advanced JavaScript (Elm, React, Angular, Backbone, Ember)

The JavaScript web framework space has exploded. Many of these are implementations of the Model, View, Controller pattern, including React, Angular, and Ember. These tools allow the creation of complex web apps (as well as mobile apps).

Web Server Operations and DNS

I learned a ton form Linux Web Operations (Safari) by Ben Whaley. "The videos discuss the relationship between web and application servers, load balancers, and databases and introduce configuration management, monitoring, containers, cryptography, and DNS."

I've struggled with DNS configuration over the years, so I watched Cricket Liu's Learning DNS series (Safari). I still wouldn't want to be responsible for a company's complex DNS infrastructure, but I can now configure my own sites DNS with a little more understanding.

Development Automation

Package Managers

It's likely that any modern web project will have some external Javascript dependencies. Package managers (analogous to Pypi or Anaconda.org on Python) have emerged to help support this. Node.js comes with the npm package manager, but Bower seems to make more sense for front-end development.1 Cody Lindley has a nice introduction to npm and Bower. Bower is well documented and easy to start using. There is a nice Flask extension to help you integrate Bower with your Python project.

Task Automation

Web development comes with lots of build-style tasks that have to happen repeatedly. For example, before you can render a webpage in the browser, you might need to convert the Less to CSS and start a local web server. Before deploying to production, you might want to also run tests and minify your Javascript.

There's a GUI application called Codekit that can do a lot of these tasks. You can also do it through a Node.js program called Grunt. I haven't used it yet, but it looks like following the documentation would be the best way to get started.

Gulp is a popular alternative to Grunt.


Visual Design

Design has never been my strong point. One way to compensate for that is to rely on the work of others. There are copious Bootstrap themes available, and some are even free.

I enjoyed Software Engineering Daily's interview with Tracy Osborn on Design for Non-designers. She has some blog posts on the topic. Tracy recommends COLOURLovers for color ideas and Font Pair for selecting fonts from Google Fonts.

User Experience Design

On the topic of UX, I finally read Steve Krug's classic Don't Make Me Think (Safari); it's great. Ginny Redish's Letting Go of Words (Safari) is similarly excellent. Steve Krug's Don't Make Me Think


I've learned a lot in the past few months. I've filled in some gaps about how CSS works. I've gotten a better grasp on the Javascript prototype model. I've learned that I can start with higher level tools (e.g. Bootstrap and JQuery) to rapidly build my side projects with some amount of visual appeal. I'm learning how to use available tools to reduce the boilerplate I have to write, automate tedious tasks, and reduce my personal technical debt.

I still have a lot of learning and a lot of practicing ahead of me, but I'm starting to feel confident that I could make headway on some of my projects. The modern frontend development landscape is massive, varied, and ever changing, but that shouldn't prohibit you from diving in if you want to.

  1. The recent buzz in package management has been about Yarn, a replacement for npm. 

Caktus GroupCome Visit Us at PyCon 2017

PyCon 2017 is fast approaching, and we’re excited to support the event this year as sponsors once again. It’s a great opportunity to meet new friends, exchange ideas and interact with the community at large.

Caktus has attended PyCon since 2010 and our developers are always excited to learn from the variety of talks scheduled for the conference. We’ll be represented by a team of 10 attendees who can’t wait to get to Portland.

Sales director Julie White and last year's prize winner

We look forward to welcoming visitors to our booth from May 18-20 to chat about building Django apps, Python best practices, or industry trends. Interested in working with Caktus or speaking with us about what it’s like to work here? Stop by during the job fair to learn more about joining Caktus as a Django Web Developer. And of course, we’ll be doing giveaways - but you’ll have to stop by and say hi to win!

In the meantime, follow Caktus on Twitter for a sneak preview of our giveaways and an early chance to win (more on this soon, so keep an eye on our feed).

Our booth is always busy, so be sure to contact us in advance to ensure dedicated time with our team.

Are you coming to PyCon this year? Let us know in the comments what talks, workshops, or events you’re most excited about.

Caktus GroupHosting Django Sites on Amazon Elastic Beanstalk


Amazon Web Services (AWS)' Elastic Beanstalk is a service that bundles up a number of their lower-level services to manage many details for you when deploying a site. We particularly like it for deploys and autoscaling.

We were first introduced to Elastic Beanstalk when taking over an existing project that used it. It's not without its shortcomings, but we've generally been happy enough with it to stick with it for the project and consider it for others.


Elastic Beanstalk can handle a number of technologies that people use to build web sites, including Python (2.7 and 3.4), Java, PHP, .Net, Node.js, and Ruby. More are probably in the works.

You can also deploy containers, with whatever you want in them.

To add a site to Elastic Beanstalk, you create a new Elastic Beanstalk application, set some configuration, and upload the source for your application. Then Elastic Beanstalk will provision the necessary underlying resources, such as EC2 (virtual machine) instances, load balancers, autoscaling groups, DNS, etc, and install your application appropriately.

Elastic Beanstalk can monitor the load and automatically scale underlying resources as needed.

When your application needs updating, you upload the updated source, and Elastic Beanstalk updates the underlying resources. You can choose from multiple update strategies.

For example, on our staging server, we have Elastic Beanstalk update the application on our existing web servers, one at a time. In production, we have Elastic Beanstalk start setting up a new set of servers, check their health, and only start directing traffic to them as they're up and healthy. Then it shuts down the previous servers.

Elastic Beanstalk and Python

As you'd expect, when deploying a Python application, Elastic Beanstalk will create a virtual environment and install whatever's in your requirements.txt file.

A lot of other configuration can be done. Here are some example configuration file snippets from Amazon's documentation on Elastic Beanstalk for Python:

    DJANGO_SETTINGS_MODULE: production.settings
    "/images/": "staticimages/"
    WSGIPath: ebdjango/wsgi.py
    NumProcesses: 3
    NumThreads: 20

    libmemcached-devel: '0.31'

    command: "django-admin.py collectstatic --noinput"
    command: "django-admin.py syncdb --noinput"
    leader_only: true
    command: "django-admin.py migrate"
    leader_only: true

Here are some things we can note:

  • We can define environment variables, like DJANGO_SETTINGS_MODULE.
  • We can tell Elastic Beanstalk to serve static files for us.
  • We ask Elastic Beanstalk to run our Django application using WSGI.
  • We can install additional packages, like memcached.
  • We can run commands during deploys.
  • Using leader_only, we can tell Elastic Beanstalk that some commands only need to be run on one instance during the deploy.

This is also where we could set parameters for autoscaling, configure the deployment strategy, set up notifications, and many other things.


The leader_only feature is great for doing something on only one of your servers during a deploy. But don't make the mistake we made of trying to use that to configure one server differently from the others, for example to run some background task periodically. Once the deploy is done, there's nothing special about the server that ran the leader_only commands during the deploy, and that server is as likely as any other to be terminated during autoscaling.

Right now, Elastic Beanstalk doesn't provide any way to readily differentiate servers so you can, for example, run things on only one server at a time. You'll have to do that yourselves.

Our solution for the situation where we ran into this was to use select_for_update to "lock" the records we were updating.


You can have Elastic Beanstalk manage RDS (Amazon's hosted database service) for you, but so far we've preferred to set up RDS outside of Elastic Beanstalk. If Elastic Beanstalk was managing it, then Elastic Beanstalk would provide pointers to the database server in environment variables, which would be more convenient than keeping track of it ourselves. On the other hand, with our database outside of Elastic Beanstalk, we know our data is safe, even if we make some terrible mistake in our Elastic Beanstalk configuration.


As always, you have to think carefully when deploying an update that includes migrations. Both of the deploy strategies we mentioned earlier will result in the migrations running on the database while some servers are still running the previous code, so you need to be sure that'll work. But that's a problem anytime you have multiple web servers and deserves a blog post of its own.

Time to deploy

One thing we're not happy with is the time it takes for an update deploy - over 20 minutes for our staging environment, and over 40 minutes for our production environment with its more conservative deploy strategy.

Some of that time is under our control. For example, it still takes longer than we'd like to set up the environment on each server, including building new environments for both Python and Node.js. We've already made some speedups there, and will continue working on that.

Other parts of the time are not under our control, especially some parts unique to production deploys. It takes AWS several minutes to provision and start a new EC2 instance, even before starting to set up our application's specific environment. It does that for one new server, waits until it's completely ready and tests healthy (several more minutes), and then starts the process all over again for the rest of the new servers it needs. (Those are done in parallel.)

When all the traffic is going to the new servers, it starts terminating the previous servers at 3 minute intervals, and waiting for all those to finish before declaring the deploy complete.

There are clear advantages to doing things this way: Elastic Beanstalk won't publish a completely broken version of our application, and the site never has any downtime during a production deploy. There are CLI tools that help you manage your deploys, plus a nice web interface to monitor what’s going on. We just wish the individual steps didn't take so long. Hopefully Elastic Beanstalk will find ways over time to improve things.


Despite some of its current shortcomings, we're quite happy to have Elastic Beanstalk in our deployment and hosting toolbox. We previously built our own tool for deploying and managing Django projects in an AWS autoscaling environment (before many of the more recent additions to the AWS suite, such as RDS for Postgres), and we know how much work it is to design, build, and maintain such a platform.

Tim HopperAutomating Python with Ansible

I wrote a few months back about how data scientists need more automation. In particular, I suggested that data scientists would be wise to learn more about automated system configuration and automated deployments.

In an attempt to take my own advice, I've finally been making myself learn Ansible. It turns out that a great way to learn it is to sit down and read through the docs, front to back; I commend that tactic to you. I also put together this tutorial to walk through a practical example of how a working data scientist might use this powerful tool.

What follows is an Ansible guide that will take you from installing Ansible to automatically deploying a long-running Python to a remote machine and running it in a Conda environment using supervisord. It presumes your development machine is on OS X and the remote machine is Debian-like; however, it shouldn't require too many changes to run it on other systems.

I wrote this post in a Jupyter notebook with a Bash kernel. You can find the notebook, Ansible files, and installation directions on my Github.


Ansible provides "human readable automation" for "app deployment" and "configuration management". Unlike tools like Chef, it doesn't require an agent to be running on remote machines. In short, it translates declarative YAML files into shell commands and runs them on your machines over SSH.

Ansible is backed by Red Hat and has a great website.

Installing Ansible with Homebrew

First, you'll need to install Ansible. On a Mac, I recommend doing this with Homebrew.

In [2]:
brew install ansible
Warning: ansible- already installed
Warning: You are using OS X 10.12.
We do not provide support for this pre-release version.
You may encounter build failures or other breakages.


Soon, I'll show you how to put write an Ansible YAML file. However, Ansible also allows you specify tasks from the command line.

Here's how we could use Ansible ping our local host:

In [3]:
ansible -i 'localhost,' -c local -m ping all
ansible -i 'localhost,' -c local -m ping all
localhost | SUCCESS => {
    "changed": false, 
    "ping": "pong"

This command calls ansible and tells it:

  • To use localhost as it's inventory (-i). Inventory is Ansible speak for machine or machines you want to be able to run commands on.
  • To connect (-c) locally (local) instead of over SSH.
  • To run the ping module (-m) to test the connection.
  • To run the command on all hosts in the inventory (in this case, our inventory is just the localhost).

Michael Booth has a post that goes into more detail about this command.

Behind the scenes, Ansible is turning this -m ping command into shell commands. (Try running with the -vvv flag to see what's happening behind the scenes.) It can also execute arbitrary commands; by default, it'll use the Bourne shell sh.

In [4]:
ansible all -i 'localhost, ' -c local -a "/bin/echo hello"

Setting up an Ansible Inventory

Instead of specifying our inventory with the -i flag each time, we should specify an Ansible inventory file. This file is a text file specifying machines you have SSH access to; you can also group machines under bracketed headings. For example:




Ansible has to be able to connect to these machines over SSH, so you will likely need to have relevant entries in your .ssh/config file.

By default, the Ansible CLI will look for a system-wide Ansible inventory file in /etc/ansible/hosts. You can also specify an alternative path for an intentory file with the -i flag.

For this tutorial, I'd like to have an inventory file specific to the project directory without having to specify it each time we call Ansible. We can do this by creating a file called ./ansible.cfg and set the name of our local inventory file:

In [5]:
cat ./ansible.cfg
cat ./ansible.cfg
inventory = ./hosts

You can check that Ansible is picking up your config file by running ansible --version.

In [6]:
ansible --version
ansible --version
  config file = /Users/tdhopper/repos/automating_python/ansible.cfg
  configured module search path = Default w/o overrides

For this example, I just have one host, a Digital Ocean VPS. To run the examples below, you should create a VPS instance on Digital Ocean, Amazon, or elsewhere; you'll want to configure it for passwordless authentication. I have an entry like this in my ~/.ssh/hosts file:

Host digitalocean
  HostName 45.55.395.23
  User root
  Port 22
  IdentityFile /Users/tdhopper/.ssh/id_rsa
  ForwardAgent yes

and my intentory file (~/hosts) is just


Before trying ansible, you should ensure that you can connect to this host:

In [7]:
ssh digitalocean echo 1
ssh digitalocean echo 1

Now I can verify that Ansible can connect to my machine by running the ping command.

In [8]:
ansible all -m ping
ansible all -m ping
digitalocean | SUCCESS => {
    "changed": false, 
    "ping": "pong"

We told Ansible to run this command on all specified hosts in the inventory. It found our inventory by loading the ansible.cfg which specified ./hosts as the inventory file.

It's possible that this will fail for you even if you can SSH into the machine. If the error is something like /bin/sh: 1: /usr/bin/python: not found, this is because your VPS doesn't have Python installed on it. You can install it with Ansible, but you may just want to manually run sudo apt-get -y install python on the VPS to get started.

Writing our first Playbook

While adhoc commands will often be useful, the real power of Ansible comes from creating repeatable sets of instructions called Playbooks.

A playbook contains a list of "plays". Each play specifies a set of tasks to be run and which hosts to run them on. A "task" is a call to an Ansible module, like the "ping" module we've already seen. Ansible comes packaged with about 1000 modules for all sorts of use cases. You can also extend it with your own modules and roles.

Our first playbook will just execute the ping module on all our hosts. It's a playbook with a single play comprised of a single task.

In [9]:
cat ping.yml
cat ping.yml
- hosts: all
  - name: ping all hosts

We can run our playbook with the ansible-playbook command.

In [10]:
ansible-playbook ping.yml
ansible-playbook ping.yml
< PLAY [all] >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

< TASK [setup] >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

ok: [digitalocean]
< TASK [ping all hosts] >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

ok: [digitalocean]
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

digitalocean               : ok=2    changed=0    unreachable=0    failed=0   

You might wonder why there are cows on your screen. You can find out here. However, the important thing is that our task was executed and returned successfully.

We can override the hosts list for the play with the -i flag to see what the output looks like when Ansible fails to run the play because it can't find the host.

Let's work now on installing the dependencies for our Python project.

Installing supervisord

"Supervisor is a client/server system that allows its users to monitor and control a number of processes on UNIX-like operating systems." We'll use it to run and monitor our Python process.

On a Debian-like system, we can install it with APT. In the Ansible DSL that's just:

- name: Install supervisord
  sudo: yes
    name: supervisor
    state: present
    update_cache: yes

You can read more about the apt module here.

Once we have it installed, we can start it with this task:

- name: Start supervisord
  sudo: yes
    name: "supervisor"
    state: running
    enabled: yes

This uses the service module.

We could add these these tasks to a playbook file (like ping.yml), but what maybe we will want to share it among multiple playbooks? For this, Ansible has a construct called Roles. A role is a collection of "variable values, certain tasks, and certain handlers – or just one or more of these things". (You can learn more about variables and handlers in the Ansible docs.)

Roles are organized as subfolders of a folder called "Roles" in the working directory. The rapid proliferation of folders in Ansible organization can be overwhelming, but a very simple rule is just a file called main.yml nestled several folders deep. In our case, it's in ./roles/supervisor/tasks/main.yml.

Check out the docs to learn more about role organization.

Here's what our roll looks like:

In [11]:
cat ./roles/supervisor/tasks/main.yml
cat ./roles/supervisor/tasks/main.yml

- name: Install supervisord
  become: true
    name: supervisor
    state: present
    update_cache: yes
- name: Start supervisord
  become: true
    name: "supervisor"
    state: running
    enabled: yes

Note that I added tags: to the task definitions. Tags just allow you to run a portion of a playbook instead of the whole thing with the --tags flag for ansible-playbook.

Now that we have the supervisor install encapsulated in a role, we can write a simple playbook to run the roll.

In [12]:
cat supervisor.yml
cat supervisor.yml
- hosts: digitalocean
    - role: supervisor
In [13]:
ansible-playbook supervisor.yml
ansible-playbook supervisor.yml
< PLAY [digitalocean] >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

< TASK [setup] >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

ok: [digitalocean]
< TASK [supervisor : Install supervisord] >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

changed: [digitalocean]
< TASK [supervisor : Start supervisord] >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

changed: [digitalocean]
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

digitalocean               : ok=3    changed=2    unreachable=0    failed=0   

Installing Conda with Ansible Galaxy

Next we want to ensure that Conda installed on our system. We could write our own role to follow the recommended process. However, Ansible has a helpful tool to help us avoid reinventing the wheel by allowing users to share roles; this is called Ansible Galaxy.

You can search the Galaxy website for miniconda and see that a handful of roles for installing Miniconda exist. I liked this one.

We can install the roll locally using the ansible-galaxy command line tool.

In [14]:
ansible-galaxy install -f andrewrothstein.miniconda

You can have the roll installed wherever you want (run ansible-galaxy install --help to see how, but by default they'll go to /usr/local/etc/ansible/roles/.

In [15]:
ls -lh /usr/local/etc/ansible/roles/andrewrothstein.miniconda
ls -lh /usr/local/etc/ansible/roles/andrewrothstein.miniconda
total 32
-rw-rw-r--  1 tdhopper  admin   1.1K Jan 16 16:52 LICENSE
-rw-rw-r--  1 tdhopper  admin   666B Jan 16 16:52 README.md
-rw-rw-r--  1 tdhopper  admin   973B Jan 16 16:52 circle.yml
drwxrwxr-x  3 tdhopper  admin   102B Mar 21 11:33 defaults
drwxrwxr-x  3 tdhopper  admin   102B Mar 21 11:33 handlers
drwxrwxr-x  4 tdhopper  admin   136B Mar 21 11:33 meta
drwxrwxr-x  3 tdhopper  admin   102B Mar 21 11:33 tasks
drwxrwxr-x  3 tdhopper  admin   102B Mar 21 11:33 templates
-rw-rw-r--  1 tdhopper  admin    57B Jan 16 16:52 test.yml
drwxrwxr-x  3 tdhopper  admin   102B Mar 21 11:33 vars

You can look at the tasks/main.yml to see the core logic of installing Miniconda. It has tasks to download the installer, run the installer, delete the installer, run conda update conda, and make conda the default system Python.

In [16]:
cat /usr/local/etc/ansible/roles/andrewrothstein.miniconda/tasks/main.yml
# tasks file for miniconda
- name: download installer...
  become: yes
  become_user: root
    url: '{{miniconda_installer_url}}'
    dest: /tmp/{{miniconda_installer_sh}}
    timeout: '{{miniconda_timeout_seconds}}'
    checksum: '{{miniconda_checksum}}'
    mode: '0755'

- name: installing....
  become: yes
  become_user: root
  command: /tmp/{{miniconda_installer_sh}} -b -p {{miniconda_parent_dir}}/{{miniconda_name}}
    creates: '{{miniconda_parent_dir}}/{{miniconda_name}}'

- name: deleting installer...
  become: yes
  become_user: root
  when: miniconda_cleanup
    path: /tmp/{{miniconda_installer_sh}}
    state: absent
- name: link miniconda...
  become: yes
  become_user: root
    dest: '{{miniconda_parent_dir}}/miniconda'
    src: '{{miniconda_parent_dir}}/{{miniconda_name}}'
    state: link

- name: conda updates
  become: yes
  become_user: root
  command: '{{miniconda_parent_dir}}/miniconda/bin/conda update -y --all'

- name: make system default python etc...
  when: miniconda_make_sys_default
  become: yes
  become_user: root
    - etc/profile.d/miniconda.sh
    src: '{{item}}.j2'
    dest: /{{item}}
    mode: 0644

Overriding Ansible Variables

Once a roll is installed locally, you can add it to a play just like you can with roles you wrote. Installing Miniconda is now as simple as:

    - role: andrewrothstein.miniconda

Before we add that to a playbook, I want to customize where miniconda is installed. If you look back at the main.yml file above, you see a bunch of things surrounded in double brackets. These are variables (in the Jinja2 template language). From the play, we can see that Miniconda will be installed at {{miniconda_parent_dir}}/{{miniconda_name}}. The role defines these variables in /andrewrothstein.miniconda/defaults/main.yml. We can override the default variables by specifying them in our play.

A play to install miniconda could look like this:

- hosts: digitalocean
    conda_folder_name: miniconda
    conda_root: /root
    - role: andrewrothstein.miniconda
      miniconda_parent_dir: "{{ conda_root }}"
      miniconda_name: "{{ conda_folder_name }}"

I added this to playbook.yml.

We now know how to use Ansible to start and run supervisord and to install Miniconda. Let's see how to use it to deploy and start our application.

Deploy Python Application

There are countless ways to deploy a Python application. We're going to see how to use Ansible to deploy from Github.

I created a little project called long_running_python_application. It has a main.py that writes a log line to stdout every 30 seconds; that's it. It also includes a Conda environment file specifying the dependencies and a shell script that activates the environment and runs the program.

We're going to use Ansible to

  1. Clone the repository into our remote machine.
  2. Create a Conda environment based on the environment.yml file.
  3. Create a supervisord file for running the program.
  4. Start the supervisord job.

Clone the repository

Cloning a repository with Ansible is easy. We just use the git module. This play will clone the repo into the specified directory. The update: yes flag tells Ansible to update the repository from the remote if it has already been cloned.

- hosts: digitalocean
    project_repo: git://github.com/tdhopper/long_running_python_process.git
    project_location: /srv/long_running_python_process
    - name: Clone project code.
        repo: "{{ project_repo }}"
        dest: "{{ project_location }}"
        update: yes

Creating the Conda Environment

Since we've now installed conda and cloned the repository with an environment.yml file, we just need to run conda env update from the directory containing the environment spec. Here's a play to do that:

- hosts: digitalocean
    project_location: /srv/long_running_python_process
    - name: Create Conda environment from project environment file.
      command: "{{conda_root}}/{{conda_folder_name}}/bin/conda env update"
        chdir: "{{ project_location }}"

It uses the command module which just executes a shell command in the desired directory.

Create a Supervisord File

By default, supervisord will look in /etc/supervisor/conf.d/ for configuration on which programs to run.

We need to put a file in there that tells supervisord to run our run.sh script. Ansible has an integrated way of setting up templates which can be placed on remote machines.

I put a supervisord job template in the ./templates folder.

In [17]:
cat ./templates/run_process.j2
cat ./templates/run_process.j2
[program:{{ program_name }}]
command=sh run.sh
directory={{ project_location }}
stderr_logfile=/var/log/{{ program_name }}.err.log
stdout_logfile=/var/log/{{ program_name }}.out.log

This is a is normal INI-style config file, except it includes Jinja2 variables. We can use the Ansible template module to create a task which fills in the variables with information about our program and copies it into the conf.d folder on the remote machine.

The play for this would look like:

- hosts: digitalocean
    project_location: /srv/long_running_python_process
    program_name: long_running_process
    supervisord_configs_path: /etc/supervisor/conf.d
    - name: Copy supervisord job file to remote
        src: ./templates/run_process.j2
        dest: "{{ supervisord_configs_path }}/run_process.conf"
        owner: root

Start the supevisord job

Finally, we just need to tell supervisord on our remote machine to start the job described by run_process.conf.

Instead of issuing our own shell commands via Ansible, we can use the supervisorctl module. The task is just:

    - name: Start job
        name: "{{ program_name }}"
        state: present

state: present ensures that Ansible calls supervisorctl reread to load a new config. Because our config has autostart=true, supervisor will start it as soon as the task is added.

The Big Playbook!

We can take everything we've described above and put it in one playbook.

This playbook will:

  • Install Miniconda using the role from Ansible Galaxy.
  • Install and start Supervisor using the role we created.
  • Clone the Github project we want to run.
  • Create a Conda environment based on the environment.yml file.
  • Create a supervisord file for running the program.
  • Start the supervisord job.

All of this will be done on the host we specify (digitalocean).

In [18]:
cat playbook.yml
cat playbook.yml
- hosts: digitalocean
    project_repo: git://github.com/tdhopper/long_running_python_process.git
    project_location: /srv/long_running_python_process
    program_name: long_running_process
    conda_folder_name: miniconda
    conda_root: /root
    supervisord_configs_path: /etc/supervisor/conf.d
    - role: andrewrothstein.miniconda
      miniconda_parent_dir: "{{ conda_root }}"
      miniconda_name: "{{ conda_folder_name }}"
    - role: supervisor
    - name: Clone project code.
        repo: "{{ project_repo }}"
        dest: "{{ project_location }}"
        update: yes
    - name: Create Conda environment from project environment file.
      command: "{{conda_root}}/{{conda_folder_name}}/bin/conda env update"
        chdir: "{{ project_location }}"
    - name: Copy supervisord job file to remote
        src: ./templates/run_process.j2
        dest: "{{ supervisord_configs_path }}/run_process.conf"
        owner: root
    - name: Start job
        name: "{{ program_name }}"
        state: present

To configure our machine, we just have to run ansible-playbook playbook.yml.

In [19]:
ANSIBLE_NOCOWS=1 ansible-playbook playbook.yml
ANSIBLE_NOCOWS=1 ansible-playbook playbook.yml

PLAY [digitalocean] ************************************************************

TASK [setup] *******************************************************************
ok: [digitalocean]

TASK [andrewrothstein.unarchive-deps : resolve platform specific vars] *********

TASK [andrewrothstein.unarchive-deps : install common pkgs...] *****************
changed: [digitalocean] => (item=[u'tar', u'unzip', u'gzip', u'bzip2'])

TASK [andrewrothstein.bash : install bash] *************************************
ok: [digitalocean]

TASK [andrewrothstein.alpine-glibc-shim : fix alpine] **************************
skipping: [digitalocean]

TASK [andrewrothstein.miniconda : download installer...] ***********************
changed: [digitalocean]

TASK [andrewrothstein.miniconda : installing....] ******************************
changed: [digitalocean]

TASK [andrewrothstein.miniconda : deleting installer...] ***********************
skipping: [digitalocean]

TASK [andrewrothstein.miniconda : link miniconda...] ***************************
changed: [digitalocean]

TASK [andrewrothstein.miniconda : conda updates] *******************************
changed: [digitalocean]

TASK [andrewrothstein.miniconda : make system default python etc...] ***********
skipping: [digitalocean] => (item=etc/profile.d/miniconda.sh) 

TASK [supervisor : Install supervisord] ****************************************
ok: [digitalocean]

TASK [supervisor : Start supervisord] ******************************************
ok: [digitalocean]

TASK [Clone project code.] *****************************************************
changed: [digitalocean]

TASK [Create Conda environment from project environment file.] *****************
changed: [digitalocean]

TASK [Copy supervisord job file to remote] *************************************
changed: [digitalocean]

TASK [Start job] ***************************************************************
changed: [digitalocean]

PLAY RECAP *********************************************************************
digitalocean               : ok=13   changed=9    unreachable=0    failed=0   

See that the PLAY RECAP shows that everything was OK, no systems were unreachable, and no tasks failed.

We can verify that the program is running without error:

In [20]:
ssh digitalocean sudo supervisorctl status
ssh digitalocean sudo supervisorctl status
long_running_process             RUNNING   pid 4618, uptime 0:01:34
In [21]:
ssh digitalocean cat /var/log/long_running_process.out.log
ssh digitalocean cat /var/log/long_running_process.out.log
INFO:root:Process ran for the 1th time
INFO:root:Process ran for the 2th time
INFO:root:Process ran for the 3th time
INFO:root:Process ran for the 4th time

If your lucky (i.e. your systems and networks were setup sufficiently similar to mine), you can run this exact same command to configure and start a process on your own system. Moreover, you could use this exact same command to start this program on an arbitrary number of machines by simply adding more hosts to your inventory and play spec!


Ansible is a powerful, customizable tool. Unlike some similar tools, it requires very little setup to start using it. As I've learned more about it, I've seen more and more ways in which I could've used it in copious projects in the past; I intend to make it a regular part of my toolkit. (Historically I've done this kind of thing with hacky combinations of shell scripts and Fabric; Ansible would often be better.)

This tutorial just scratches the surface of the Ansible functionality. If you want to learn more, I again recommend reading through the docs; they're very good. Of course, you should start writing and running your own playbooks as soon as possible! I also liked this tutorial from Server Admin for Programmers. If you want to compare Ansible to alternatives, the Taste Test book by Matt Jaynes looks promising. For more on Supervisor, serversforhackers.com has a nice tutorial, and its docs are thorough.

Caktus GroupA Production-ready Dockerfile for Your Python/Django App

Docker has matured a lot since it was released nearly 4 years ago. We’ve been watching it closely at Caktus, and have been thrilled by the adoption -- both by the community and by service providers. As a team of Python and Django developers, we’re always searching for best of breed deployment tools. Docker is a clear fit for packaging the underlying code for many projects, including the Python and Django apps we build at Caktus.

Technical overview

There are many ways to containerize a Python/Django app, no one of which could be considered “the best.” That being said, I think the following approach provides a good balance of simplicity, configurability, and container size. The specific tools I’ll be using are: Docker (of course), Alpine Linux, and uWSGI.

Alpine Linux is a simple, lightweight Linux distribution based on musl libc and Busybox. Its main claim to fame on the container landscape is that it can create a very small (5MB) Docker image. Typically one’s application will be much larger than that after the code and all dependencies have been included, but the container will still be much smaller than if based on a general-purpose Linux distribution.

There are many WSGI servers available for Python, and we use both Gunicorn and uWSGI at Caktus. A couple of the benefits of uWSGI are that (1) it’s almost entirely configurable through environment variables (which fits well with containers), and (2) it includes native HTTP support, which can circumvent the need for a separate HTTP server like Apache or Nginx, provided static files are hosted on a 3rd-party CDN such as Amazon S3.

The Dockerfile

Without further ado, here’s a production-ready Dockerfile you can use as a starting point for your project (it should be added in your top level project directory, or whichever directory contains the Python package(s) provided by your application):

FROM python:3.5-alpine

# Copy in your requirements file
ADD requirements.txt /requirements.txt

# OR, if you’re using a directory for your requirements, copy everything (comment out the above and uncomment this if so):
# ADD requirements /requirements

# Install build deps, then run `pip install`, then remove unneeded build deps all in a single step. Correct the path to your production requirements file, if needed.
RUN set -ex \
    && apk add --no-cache --virtual .build-deps \
            gcc \
            make \
            libc-dev \
            musl-dev \
            linux-headers \
            pcre-dev \
            postgresql-dev \
    && pyvenv /venv \
    && /venv/bin/pip install -U pip \
    && LIBRARY_PATH=/lib:/usr/lib /bin/sh -c "/venv/bin/pip install --no-cache-dir -r /requirements.txt" \
    && runDeps="$( \
            scanelf --needed --nobanner --recursive /venv \
                    | awk '{ gsub(/,/, "\nso:", $2); print "so:" $2 }' \
                    | sort -u \
                    | xargs -r apk info --installed \
                    | sort -u \
    )" \
    && apk add --virtual .python-rundeps $runDeps \
    && apk del .build-deps

# Copy your application code to the container (make sure you create a .dockerignore file if any large files or directories should be excluded)
RUN mkdir /code/
WORKDIR /code/
ADD . /code/

# uWSGI will listen on this port

# Add any custom, static environment variables needed by Django or your settings file here:
ENV DJANGO_SETTINGS_MODULE=my_project.settings.deploy

# uWSGI configuration (customize as needed):

# Call collectstatic (customize the following line with the minimal environment variables needed for manage.py to run):
RUN DATABASE_URL=none /venv/bin/python manage.py collectstatic --noinput

# Start uWSGI
CMD ["/venv/bin/uwsgi", "--http-auto-chunked", "--http-keepalive"]

We extend from the Alpine flavor of the official Docker image for Python 3.5, copy the folder containing our requirements files to the container, and then, in a single line, (a) install the OS dependencies needed, (b) pip install the requirements themselves (edit this line to match the location of your requirements file, if needed), (c) scan our virtual environment for any shared libraries linked to by the requirements we installed, and (d) remove the C compiler and any other OS packages no longer needed, except those identified in step (c) (this approach, using scanelf, is borrowed from the underlying 3.5-alpine Dockerfile). It’s important to keep this all on one line so that Docker will cache the entire operation as a single layer.

You’ll notice I’ve only included a minimal set of OS dependencies here. If this is an established production app, you’ll most likely need to visit https://pkgs.alpinelinux.org/packages, search for the Alpine Linux package names of the OS dependencies you need, including the -dev supplemental packages as needed, and add them to the list above.

Next, we copy our application code to the image, set some default environment variables, and run collectstatic. Be sure to change the values for DJANGO_SETTINGS_MODULE and UWSGI_WSGI_FILE to the correct paths for your application (note that the former requires a Python package path, while the latter requires a file system path). In the event you’re not serving static media directly from the container (e.g., with Whitenoise), the collectstatic command can also be removed.

Finally, the --http-auto-chunked and --http-keepalive options to uWSGI are needed in the event the container will be hosted behind an Amazon Elastic Load Balancer (ELB), because Django doesn’t set a valid Content-Length header by default, unless the ConditionalGetMiddleware is enabled. See the note at the end of the uWSGI documentation on HTTP support for further detail.

Building and testing the container

Now that you have the essentials in place, you can build your Docker image locally as follows:

docker build -t my-app .

This will go through all the commands in your Dockerfile, and if successful, store an image with your local Docker server that you could then run:

docker run -e DATABASE_URL=none -t my-app

This command is merely a smoke test to make sure uWSGI runs, and won’t connect to a database or any other external services.

Running commands during container start-up

As an optional final step, I recommend creating an ENTRYPOINT script to run commands as needed during container start-up. This will let us accomplish any number of things, such as making sure Postgres is available or running migrate or collectstatic during container start-up. Save the following to a file named docker-entrypoint.sh in the same directory as your Dockerfile:

set -e

until psql $DATABASE_URL -c '\l'; do
  >&2 echo "Postgres is unavailable - sleeping"
  sleep 1

>&2 echo "Postgres is up - continuing"

if [ "x$DJANGO_MANAGEPY_MIGRATE" = 'xon' ]; then
    /venv/bin/python manage.py migrate --noinput

if [ "x$DJANGO_MANAGEPY_COLLECTSTATIC" = 'xon' ]; then
    /venv/bin/python manage.py collectstatic --noinput

exec "$@"

Next, add the following line to your Dockerfile, just above the CMD statement:

ENTRYPOINT ["/code/docker-entrypoint.sh"]

This will (a) make sure a database is available (usually only needed when used with Docker Compose), (b) run outstanding migrations, if any, if the DJANGO_MANAGEPY_MIGRATE is set to on in your environment, and (c) run collectstatic if DJANGO_MANAGEPY_COLLECTSTATIC is set to on in your environment. Even if you add this entrypoint script as-is, you could still choose to run migrate or collectstatic in separate steps in your deployment before releasing the new container. The only reason you might not want to do this is if your application is highly sensitive to container start-up time, or if you want to avoid any database calls as the container starts up (e.g., for local testing). If you do rely on these commands being run during container start-up, be sure to set the relevant variables in your container’s environment.

Creating a production-like environment locally with Docker Compose

To run a complete copy of production services locally, you can use Docker Compose. The following docker-compose.yml will create a barebones, ephemeral, AWS-like container environment with Postgres and Redis for testing your production environment locally.

This is intended for local testing of your production environment only, and will not save data from stateful services like Postgres upon container shutdown.

version: '2'

      POSTGRES_DB: app_db
      POSTGRES_USER: app_user
      POSTGRES_PASSWORD: changeme
    restart: always
    image: postgres:9.6
      - "5432"
    restart: always
    image: redis:3.0
      - "6379"
      DATABASE_URL: postgres://app_user:changeme@db/app_db
      REDIS_URL: redis://redis
      context: .
      dockerfile: ./Dockerfile
      - db:db
      - redis:redis
      - "8000:8000"

Copy this into a file named docker-compose.yml in the same directory as your Dockerfile, and then run:

docker-compose up --build -d

This downloads (or builds) and starts the three containers listed above. You can view output from the containers by running:

docker-compose logs

If all services launched successfully, you should now be able to access your application at http://localhost:8000/ in a web browser.

Extra: Blocking Invalid HTTP_HOST header errors with uWSGI

To avoid Django’s Invalid HTTP_HOST header errors (and prevent any such spurious requests from taking up any more CPU cycles than absolutely necessary), you can also configure uWSGI to return an HTTP 400 response immediately without ever invoking your application code. This can be accomplished by adding a command line option to uWSGI in your Dockerfile script, e.g., --route-host=’^(?!www.myapp.com$) break:400' (note, the single quotes are required here, to prevent the shell from attempting to interpret the regular expression). If preferred (for example, in the event you use a different domain for staging and production), you can accomplish the same end by setting an environment variable via your hosting platform: UWSGI_ROUTE_HOST=‘^(?!www.myapp.com$) break:400'.

That concludes this high-level introduction to containerizing your Python/Django app for hosting on AWS Elastic Beanstalk (EB), Elastic Container Service (ECS), or elsewhere. Each application and Dockerfile will be slightly different, but I hope this provides a good starting point for your containers. Shameless plug: If you’re looking for a simple (and at least temporarily free) way to test your Docker containers on AWS using an Elastic Beanstalk Multicontainer Docker environment or the Elastic Container Service, checkout AWS Container Basics (more on this soon). Good luck!

Update 1 (March 31, 2017): There is no need for depends_on in container definitions that already include links. This has been removed. Thanks Anderson Lima for the tip!

Update 2 (March 31, 2017): Adding --no-cache-dir to the pip install command saves a additional disk space, as this prevents pip from caching downloads and caching wheels locally. Since you won't need to install requirements again after the Docker image has been created, this can be added to the pip install command. The post has been updated. Thanks Hemanth Kumar for the tip!

Tim HopperSome Reflections on Being Turned Down for a Lot of Data Science Jobs

👉 The decision was close, but the team has decided to keep looking for someone who might have more direct neural net experience.

👉 Honestly, I think the way you communicated your thought process and results was confusing for some people in the room.

👉 He's needing someone with an image analysis background for data scientist we're hiring now.

👉 Quite honestly given your questions [about vacation policy] and the fact that you are considering other options, [we] may not be the best choice for you.

These quotes above are some of the reasons I've been given for why I wasn't offered a data science job after interviewing. I've been told a variety of other reasons as well: company decided against hiring remotes after interviewing (I've heard this at least 3 times), company thought I changed jobs too frequently, company decided it didn't have necessary data infrastructure in place for data science work. Multiple companies gave no particular reason; some of these were at least kind enough to notify me they weren't interested. One company hired someone with a Ph.D. from MIT soon after turning me down.

In the last five years, I've clearly interviewed for a lot of data science jobs, and I've also been turned down for a lot of data science jobs. I've spent a good bit of time reflecting on why I wasn't offered this job or that. Several folks have asked me if I had any advice to share on the experience, and I hope to offer that here.

You never really know

I learned with graduate school applications years ago: you rarely truly know why you were turned down. Maybe my GRE scores weren't high enough, or maybe the reviewer rushed through my application in the 5 minutes before lunch. Maybe my statement of interest was too weak, or maybe the department needed to accept an alumni's child.

The same goes for companies. I'm fairly skeptical that the reasons I have been given for why I was passed by are the full story, and I suspect you will rarely (if ever) know the real reasons why you weren't offered a job. I try to use the reasons I hear as a way to help me refine my skills and better present myself, but I don't put too much weight in them.

Some advice anyway

That said, here are a few takeaways from interviewing for probably 20 data science jobs since 2012.

  • Companies often use interviews as a time to figure out what they're really looking for. I suspect this rarely intentional. But actually interviewing candidates forces a team to talk through what they're actually looking for, and they often realize they had differing perspectives prior to the interview.
  • Companies where "data science" is a new addition need your help in understanding what data science can do for them. As much as possible, use the interview to sell your vision for what data science can offer at the company, how you'll get it off the ground, and what the ROI might be.
  • Being the wrong fit for what a company needs is not ideal. I've come to appreciate a company trying to ensure my abilities align with their needs. You'd hope this was always the case, but I've been hired when it wasn't. That said, I hesitate to say you should always look for this: if you need a job, and someone offers you a job, you should feel free to take it!
  • Data infrastructure is important and many companies are lacking it. Many data scientists can attest to being hired at a company only to discover the data they needed wasn't available, and they spent months or years building the tools required for them to start their analysis. Many companies are naive about how much engineering effort is required for effective data science. Don't assume that a company with a grand vision for data science necessarily knows what it will take to accomplish that vision.
  • Many companies are still uneasy about data science being done remotely. I think this is silly, but I'm biased.
  • There's little consistency as to what you might be asked in a data science interview. I've been asked about Java design patterns, how to solve combinatorics problems, to describe my favorite machine learning model, to explain the SMO algorithm, my opinions about the TensorFlow API, how I do software testing, to analyze a never-before-seen dataset and prepare a presentation in a 4 hour window, the list goes on. I spent a flight to the west coast reading up on the statistics of A/B testing only to be asked largely soft-skills type questions for an entire interview. I've largely given up attempting any special preparation for interviews.
  • Networking is still king. Hiring is hard, and interviewing is hard; having a prior relationship with an applicant is attractive and reduces hiring uncertainty. In my own experience, my friendships and connections with the data science community on Twitter has shaped my career. Don't downplay the benefits of networking.


So how do you get a data science job? I don't know.

I've been unbelievably fortunate to be continuously employed since college, but I'm not sure how to tell you to repeat that. The best I have to offer is to reiterate the conclusion of my recent talk about data science as a career. Learn and know the hard stuff: linear algebra, probability, statistics, machine learning, math modeling, data structures, algorithms, distributed systems, etc. You probably won't use this knowledge every day in your job, but interviewers love to ask about it anyway.

At the same time, don't forget about the even harder skills: communication, careful thought, prose writing skill, software writing skill, software engineering, tenacity, Stack Overflow. You will use these every day in your job, and they'll help you present yourself well in an interview.1

Further Reading

  1. With the exception of Stack Overflow. Using Stack Overflow in an interview is strangely taboo. 

Philip SemanchukThanks to PYPTUG!

The logo of the Python Piedmont Triad Users Group
Thanks to PYPTUG, the Python Piedmont Triad Users Group, for inviting me to speak at their monthly meeting last night!

I gave a slightly expanded version of the talk I gave at PyData Carolinas 2016 about connecting Python to compiled languages like C, C++, and Fortran. (Slides from that talk are here.) I appreciate the time and attention of everyone who attended last night, especially Francois Dion for organizing and reminding us of some of the interesting new things in Python 3.5 and 3.6.

Last night’s talk wasn’t recorded, but you can see the version of the talk I gave at PyData at https://www.youtube.com/watch?v=aUSokzzsEko , or you can watch the embedded version below.


Caktus GroupOpening External Links: Same Tab or New?

The Debate

My teammates and I recently engaged in a spirited debate over whether outbound links (links to external websites) should open in the same or in a new tab. “Same tab” was a default behavior for a set of external links on a project we were working on. A suggestion had been made, however, that the behavior be changed.

Two main arguments emerged: some of us felt that opening outbound links in a new tab was a behavior so well established that most users expected it, and it was therefore a better experience. Others suggested that users who prefer a “new tab” behavior can always opt for it (by using a right-click or a keyboard shortcut), but that forcing this behavior on all users constituted hijacking their browsing experience.

In Search of Answers on WWW

Reminding myself that my own experience is not the experience of the users for whom we build web applications, I set out to search for information available on the topic online, and then conducted guerrilla usability testing with a small group of users similar to the user base of the website around which the debate had started.

My online research revealed the following:

  • Marketing communities advocate for opening external links in a new tab. The primary argument is to prevent users from leaving the website. An increased page bounce rate can negatively affect page ranking, hence allowing people to leave a page is considered bad SEO practice.
  • User experience and web development communities advocate for opening external links in the same tab. The arguments include:
    • Anything that takes control away from the user is bad experience; users should be able to decide whether or not they want to open a link in a new tab;
    • If users do not know that a given link will take them to an external website, they may get disoriented when the website opens in a new tab;
    • Opening any link in a new tab may present an accessibility issue (it breaks the workflow for users who browse the web leveraging assistive technologies)
    • Opening a new tab on a smartphone makes it difficult for the user to return to the original website
  • User opinions I have found on the Internet included preferences for either behavior:
    • Some users prefer the “new tab” behavior. That is usually the case when external links serve as reference material in support of the main article a user may be reading. In those cases opening external links in a new tab makes it easier for the user to return to the original article and to continue reading. It becomes even more important if the user continues browsing deeper by following links from external websites they have already opened. Opening external links in the same tab may, in those cases, lead to a so-called “back-button fatigue” if the user is forced to click the browser's back button multiple times in order to return to the website where their browsing began.
    • Some users prefer the “same tab” behavior. They find the experience to be seamless when they can browse back and forth between external websites and the website of origin by clicking the browser’s back button. They also argue that with the default behavior set to “same tab,” users who prefer “new tab” behavior can still achieve it through a keyboard shortcut or a right-click. Users who prefer “same tab” behavior, however, have no recourse if the “new tab” behavior is imposed on them.
  • Within UX and accessibility communities, some proponents of the “same tab” behavior are willing to make an exception and allow for external links to open in a new tab as long as outbound links are clearly marked as external (for example, through a use of text or an appropriate icon).
  • There is a security concern related to opening links in a new tab. A vulnerability of target=”_blank” attribute may leave users open to a phishing attack unless the target=”_blank”attribute is accompanied by rel="noopener" attribute.

Informal Poll Results

In addition to researching information published online, I also polled my peers in UX online communities. While I found a strong, collective, professional advocacy for the “same tab” behavior, individual preferences of UX professionals as users were split between the two behaviors. People cited the same arguments in favor or against either behavior that I summarized based on my Internet search.

Guerrilla User Testing

In the end, for us UX-ers, it’s all about the user. Given the range of opinions on the matter, I thought a quick and dirty usability test would help answer the question about what makes sense to the type of user that visits the website my teammates and I were working on.

I tested five users. Three out of five either expressed an expectation of or demonstrated a preference for external web pages getting opened in a new tab. Two of them explicitly chose to open outbound links in a new tab once they realized that by default the links opened pages in the same tab. None of these users were bothered by the “same tab” behavior. They did, however, note that on many other websites external links open pages in a new tab by default. The remaining two users reported not thinking about or even noticing that the pages opened in the same tab. All five users browsed seamlessly from linked pages back to the original page using the browser’s back button.


The qualitative results of my inquiry suggest a split of preferences among users between the “same tab” and “new tab” behaviors. From the user experience perspective, the strongest (in my opinion) arguments in favor of opening outbound links in the same tab lie in accessibility and mobile use considerations. For most stakeholders, I suspect, the SEO argument outweighs any reasoning that stems from user experience. However, as the push for building accessible websites increases as does the rate at which users access content on mobile devices, stakeholders may find themselves between a rock and a hard place searching to strike a balance between attracting new audience through SEO efforts and retaining existing users by tending to their accessibility and mobile browsing needs.

Finally, marketing as well as UX and web development communities may consider giving up the struggle for a final answer. A decision about opening external links in the same or in a new tab may have to be made on a project by project basis by finding the right balance between business and user value-add.

External resources

(Websites linked below will open in a new tab.)

Caktus GroupPython type annotations

When it comes to programming, I have a belt and suspenders philosophy. Anything that can help me avoid errors early is worth looking into.

The type annotation support that's been gradually added to Python is a good example. Here's how it works and how it can be helpful.


The first important point is that the new type annotation support has no effect at runtime. Adding type annotations in your code has no risk of causing new runtime errors: Python is not going to do any additional type-checking while running.

Instead, you'll be running separate tools to type-check your programs statically during development. I say "separate tools" because there's no official Python type checking tool, but there are several third-party tools available.

So, if you chose to use the mypy tool, you might run:

$ mypy my_code.py

and it might warn you that a function that was annotated as expecting string arguments was going to be called with an integer.

Of course, for this to work, you have to be able to add information to your code to let the tools know what types are expected. We do this by adding "annotations" to our code.

One approach is to put the annotations in specially-formatted comments. The obvious advantage is that you can do this in any version of Python, since it doesn't require any changes to the Python syntax. The disadvantages are the difficulties in writing these things correctly, and the coincident difficulties in parsing them for the tools.

To help with this, Python 3.0 added support for adding annotations to functions (PEP-3107), though without specifying any semantics for the annotations. Python 3.6 adds support for annotations on variables (PEP-526).

Two additional PEPs, PEP-483 and PEP-484, define how annotations can be used for type-checking.

Since I try to write all new code in Python 3, I won't say any more about putting annotations in comments.

Getting started

Enough background, let's see what all this looks like.

Python 3.6 was just released, so I’ll be using it. I'll start with a new virtual environment, and install the type-checking tool mypy (whose package name is mypy-lang).:

$ virtualenv -p $(which python3.6) try_types
$ . try_types/bin/activate
$ pip install mypy-lang

Let's see how we might use this when writing some basic string functions. Suppose we're looking for a substring inside a longer string. We might start with:

def search_for(needle, haystack):
    offset = haystack.find(needle)
    return offset

If we were to call this with anything that's not text, we'd consider it an error. To help us avoid that, let's annotate the arguments:

def search_for(needle: str, haystack: str):
    offset = haystack.find(needle)
    return offset

Does Python care about this?:

$ python search1.py

Python is happy with it. There's not much yet for mypy to check, but let's try it:

$ mypy search1.py

In both cases, no output means everything is okay.

(Aside: mypy uses information from the files and directories on its command line plus all packages they import, but it only does type-checking on the files and directories on its command line.)

So far, so good. Now, let's call our function with a bad argument by adding this at the end:

search_for(12, "my string")

If we tried to run this, it wouldn't work:

$ python search2.py
Traceback (most recent call last):
    File "search2.py", line 4, in <module>
        search_for(12, "my string")
    File "search2.py", line 2, in search_for
        offset = haystack.find(needle)
TypeError: must be str, not int

In a more complicated program, we might not have run that line of code until sometime when it would be a real problem, and so wouldn't have known it was going to fail. Instead, let's check the code immediately:

$ mypy search2.py
search2.py:4: error: Argument 1 to "search_for" has incompatible type "int"; expected "str"

Mypy spotted the problem for us and explained exactly what was wrong and where.

We can also indicate the return type of our function:

def search_for(needle: str, haystack: str) -> str:
    offset = haystack.find(needle)
    return offset

and ask mypy to check it:

$ mypy search3.py
search3.py: note: In function "search_for":
search3.py:3: error: Incompatible return value type (got "int", expected "str")

Oops, we're actually returning an integer but we said we were going to return a string, and mypy was smart enough to work that out. Let's fix that:

def search_for(needle: str, haystack: str) -> int:
    offset = haystack.find(needle)
    return offset

And see if it checks out:

$ mypy search4.py

Now, maybe later on we forget just how our function works, and try to use the return value as a string:

x = len(search_for('the', 'in the string'))

Mypy will catch this for us:

$ mypy search5.py
search5.py:5: error: Argument 1 to "len" has incompatible type "int"; expected "Sized"

We can't call len() on an integer. Mypy wants something of type Sized -- what's that?

More complicated types

The built-in types will only take us so far, so Python 3.5 added the typing module, which both gives us a bunch of new names for types, and tools to build our own types.

In this case, typing.Sized represents anything with a __len__ method, which is the only kind of thing we can call len() on.

Let's write a new function that'll return a list of the offsets of all of the instances of some string in another string. Here it is:

from typing import List

def multisearch(needle: str, haystack: str) -> List[int]:
    # Not necessarily the most efficient implementation
    offset = haystack.find(needle)
    if offset == -1:
        return []
    return [offset] + multisearch(needle, haystack[offset+1:])

Look at the return type: List[int]. You can define a new type, a list of a particular type of elements, by saying List and then adding the element type in square brackets.

There are a number of these - e.g. Dict[keytype, valuetype] - but I'll let you read the documentation to find these as you need them.

mypy passed the code above, but suppose we had accidentally had it return None when there were no matches:

def multisearch(needle: str, haystack: str) -> List[int]:
    # Not necessarily the most efficient implementation
    offset = haystack.find(needle)
    if offset == -1:
        return None
    return [offset] + multisearch(needle, haystack[offset+1:])

mypy should spot that there's a case where we don't return a list of integers, like this:

$ mypy search6.py

Uh-oh - why didn't it spot the problem here? It turns out that by default, mypy considers None compatible with everything. To my mind, that's wrong, but luckily there's an option to change that behavior:

$ mypy --strict-optional search6.py
search6.py: note: In function "multisearch":
search6.py:7: error: Incompatible return value type (got None, expected List[int])

I shouldn't have to remember to add that to the command line every time, though, so let's put it in a configuration file just once. Create mypy.ini in the current directory and put in:

strict_optional = True

And now:

$ mypy search6.py
search6.py: note: In function "multisearch":
search6.py:7: error: Incompatible return value type (got None, expected List[int])

But speaking of None, it's not uncommon to have functions that can either return a value or None. We might change our search_for method to return None if it doesn't find the string, instead of -1:

def search_for(needle: str, haystack: str) -> int:
    offset = haystack.find(needle)
    if offset == -1:
        return None
        return offset

But now we don't always return an int and mypy will rightly complain:

$ mypy search7.py
search7.py: note: In function "search_for":
search7.py:4: error: Incompatible return value type (got None, expected "int")

When a method can return different types, we can annotate it with a Union type:

from typing import Union

def search_for(needle: str, haystack: str) -> Union[int, None]:
    offset = haystack.find(needle)
    if offset == -1:
        return None
        return offset

There's also a shortcut, Optional, for the common case of a value being either some type or None:

from typing import Optional

def search_for(needle: str, haystack: str) -> Optional[int]:
    offset = haystack.find(needle)
    if offset == -1:
        return None
        return offset

Wrapping up

I've barely touched the surface, but you get the idea.

One nice thing is that the Python libraries are all annotated for us already. You might have noticed above that mypy knew that calling find on a str returns an int - that's because str.find is already annotated. So you can get some benefit just by calling mypy on your code without annotating anything at all -- mypy might spot some misuses of the libraries for you.

For more reading:

Tim HopperLogistic Regression Rules Everything Around Me

Fred Benenson spent 6 years doing data science at Kickstarter. When he left last year, he wrote a fantastic recap of his experience.

His "list of things I've discovered over the years" is particularly good. Here are a few of the things that resonated with me:

  • The more you can work with someone to help refine their question the easier it will be to answer
  • Conducting a randomized controlled experiment via an A/B test is always better than analyzing historical data
  • Metrics are crucial to the story a company tells itself; it is essential to honestly and rigorously define them
  • Good experimental design is difficult; don't allow a great testing framework to let you get lazy with it
  • Data science (A/B testing, etc.) can help you how to optimize for a particular outcome, but it will never tell you which particular outcome to optimize for
  • Always seek to record and attain data in its rawest form, whether you're instrumenting something yourself or retrieving it from an API
  • I highly recommend reading the whole post.

    Philip SemanchukPandas Surprise


    Part of learning how to use any tool is exploring its strengths and weaknesses. I’m just starting to use the Python library Pandas, and my naïve use of it exposed a weakness that surprised me.


    A photo of the many shapes and colors in Lucky Charms cerealThanks to bradleypjohnson for sharing this Lucky Charms photo under CC BY 2.0.

    I have a long list of objects, each with the properties “color” and “shape”. I want to count the frequency of each color/shape combination. A sample of what I’m trying to achieve could be represented in a grid like this –

           circle square star
    blue        8     41   18
    orange      5     33   25
    red        53     64   58

    At first I implemented this with a dictionary of collections.Counter instances where the top level dictionary is keyed by shape, like so –

    import collections
    SHAPES = ('square', 'circle', 'star', )
    frequencies = {shape: collections.Counter() for shape in SHAPES}

    Then I counted my frequencies using the code below. (For simplicity, assume that my objects are simple 2-tuples of (shape, color)).

    for shape, color in all_my_objects:
        frequencies[shape][color] += 1

    So far, so good.

    Enter the Pandas

    This looked to me like a perfect opportunity to use a Pandas DataFrame which would nicely support the operations I wanted to do after tallying the frequencies, like adding a column to represent the total number (sum) of instances of each color.

    It was especially easy to try out a DataFrame because my counting loop ( for...all_my_objects) wouldn’t change, only the definition of frequencies. (Note that the code below requires I know in advance all the possible colors I can expect to see, which the Dict + Counter version does not. This isn’t a problem for me in my real-world application.)

    import pandas as pd
    frequencies = pd.DataFrame(columns=SHAPES, index=COLORS, data=0,
    for shape, color in all_my_objects:
        frequencies[shape][color] += 1

    It Works, But…

    Both versions of the code get the job done, but using the DataFrame as a frequency counter turned out to be astonishingly slow. A DataFrame is simply not optimized for repeatedly accessing individual cells as I do above.

    How Slow is it?

    To isolate the effect pandas was having on performance, I used Python’s timeit module to benchmark some simpler variations on this code. In the version of Python I’m using (3.6), the default number of iterations for each timeit test is 1 million.

    First, I timed how long it takes to increment a simple variable, just to get a baseline.

    Second, I timed how long it takes to increment a variable stored inside a collections.Counter inside a dict. This mimics the first version of my code (above) for a frequency counter. It’s more complex than the simple variable version because Python has to resolve two hash table references (one inside the dict, and one inside the Counter). I expected this to be slower, and it was.

    Third, I timed how long it takes to increment one cell inside a 2×2 NumPy array. Since Pandas is built atop NumPy, this gives an idea of how the DataFrame’s backing store performs without Pandas involved.

    Fourth, I timed how long it takes to increment one cell inside a 2×2 Pandas DataStore. This is what I had used in my real code.

    Raw Benchmark Results

    Here’s what timeit showed me. Sorry for the cramped formatting.

    $ python
     Python 3.6.0 (v3.6.0:41df79263a11, Dec 22 2016, 17:23:13)
     [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
     Type "help", "copyright", "credits" or "license" for more information.
     >>> import timeit
     >>> timeit.timeit('data += 1', setup='data=0')
     >>> timeit.timeit('data[0][0]+=1',setup='from collections import Counter;data={0:Counter()}')
     >>> timeit.timeit('data[0][0]+=1',setup='import numpy as np;data=np.zeros((2,2))')
     >>> timeit.timeit('data[0][0]+=1',setup='import pandas as pd;data=pd.DataFrame(data=[[0,0],[0,0]],dtype="int")')

    Benchmark Results Summary

    Here’s a summary of the results from above (decimals truncated at 3 digits). The rightmost column shows the results normalized so the fastest method (incrementing a simple variable) equals 1.

    Actual (seconds) Normalized (seconds)
    Simple variable 0.092 1
    Dict + Counter 0.683 7.398
    Numpy 2D array 0.890 9.639
    Pandas DataFrame 157.564 1704.784

    As you can see, resolving the index references in the middle two cases (Dict + Counter in one case, NumPy array indices in the other) slows things down, which should come as no surprise. The NumPy array is a little slower than the Dict + Counter.

    The DataFrame, however, is about 150 – 200 times slower than either of those two methods. Ouch!

    I can’t really even give you a graph of all four of these methods together because the time consumed by the DataFrame throws the chart scale out of whack.

    Here’s a bar chart of the first three methods –

    A bar chart of the first three methods in the preceding table

    Here’s a bar chart of all four –

    A bar chart of all four methods in the preceding table

    Why Is My DataFrame Access So Slow?

    One of the nice features of DataFrames is that they support dictionary-like labels for rows and columns. For instance, if I define my frequencies to look like this –

    >>> SHAPES = ('square', 'circle', 'star', )
    >>> COLORS = ('red', 'blue', 'orange')
    >>> pd.DataFrame(columns=SHAPES, index=COLORS, data=0, dtype='int')
            square  circle  star
    red          0       0     0
    blue         0       0     0
    orange       0       0     0

    Then frequencies['square']['orange'] is a valid reference.

    Not only that, DataFrames support a variety of indexing and slicing options including –

    • A single label, e.g. 5 or 'a'
    • A list or array of labels ['a', 'b', 'c']
    • A slice object with labels 'a':'f'
    • A boolean array
    • A callable function with one argument

    Here are those techniques applied in order to the frequencies DataFrame so you can see how they work –

    >>> frequencies['star']
    red       0
    blue      0
    orange    0
    Name: star, dtype: int64
    >>> frequencies[['square', 'star']]
            square  star
    red          0     0
    blue         0     0
    orange       0     0
    >>> frequencies['red':'blue']
          square  circle  star
    red        0       0     0
    blue       0       0     0
    >>> frequencies[[True, False, True]]
            square  circle  star
    red          0       0     0
    orange       0       0     0
    >>> frequencies[lambda x: 'star']
    red       0
    blue      0
    orange    0
    Name: star, dtype: int64

    This flexibility has a price. Slicing (which is what is invoked by the square brackets) calls an object’s __getitem__() method. The parameter to __getitem__()  is the whatever was inside the square brackets. A DataFrame’s __getitem__() has to figure out what the passed parameter represents. Determining whether the parameter is a label reference, a callable, a boolean array, or something else takes time.

    If you look at the DataFrame’s __getitem__() implementation, you can see all the code that has to execute to resolve a reference. (I linked to the version of the code that was current when I wrote this in February of 2017. By the time you read this, the actual implementation may differ.) Not only does __getitem__() have a lot to do, but because I’m accessing a cell (rather than a whole row or column), there’s two slice operations, so __getitem__() gets invoked twice each time I increment my counter.

    This explains why the DataFrame is so much slower than the other methods. The dictionary and Counter both only support key lookup in a hash table, and a NumPy array has far fewer slicing options than a DataFrame, so its __getitem__() implementation can be much simpler.

    Better DataFrame Indexing?

    DataFrames support a few methods that exist explicitly to support “fast” getting and setting of scalars. Those methods are .at() (for label lookups) and .iat() (for integer-based index lookups). It also provides get_value() and set_value(), but those methods are deprecated in the version I have (0.19.2).

    “Fast” is how the Panda’s documentation describes these methods. Let’s use timeit to get some hard data. I’ll try at() and iat(); I’ll also try get_value()/set_value() even though they’re deprecated.

    >>> timeit.timeit("data.at['red','square']+=1",setup="import pandas as pd;data=pd.DataFrame(columns=('square','circle','star'),index=('red','blue','orange'),data=0,dtype='int')")
    >>> timeit.timeit('data.iat[0,0]+=1',setup='import pandas as pd;data=pd.DataFrame(data=[[0,0],[0,0]],dtype="int")')
    >>> timeit.timeit('data.set_value(0,0,data.get_value(0,0)+1)',setup='import pandas as pd;data=pd.DataFrame(data=[[0,0],[0,0]],dtype="int")')

    These methods are better, but they’re still pretty bad. Let’s put those numbers in context by comparing them to other techniques. This time, for normalized results, I’m going to use my Dict + Counter method as the baseline of 1 and compare all other methods to that. The row “DataFrame (naïve)” refers to naïve slicing, like frequencies[0][0].

    Actual (seconds) Normalized (seconds)
    Dict + Counter 0.683 1
    Numpy 2D array 0.890 1.302
    DataFrame (get/set) 15.050 22.009
    DataFrame (at) 36.331 53.130
    DataFrame (iat) 42.015 61.441
    DataFrame (naïve) 157.564 230.417

    The best I can do with a DataFrame uses deprecated methods, and is still over 20 times slower than the Dict + Counter. If I use non-deprecated methods, it’s over 50 times slower.


    I like label-based access to my frequency counters, I like the way I can manipulate data in a DataFrame (not shown here, but it’s useful in my real-world code), and I like speed. I don’t necessarily need blazing fast speed, I just don’t want slow.

    I can have my cake and eat it too by combining methods. I do my counting with the Dict + Counter method, and use the result as initialization data to a DataFrame constructor.

    SHAPES = ('square', 'circle', 'star', )
    frequencies = {shape: collections.Counter() for shape in SHAPES}
    for shape, color in all_my_objects:
        frequencies[shape][color] += 1
    frequencies = pd.DataFrame(data=frequencies)

    The frequencies DataFrame now looks something like this –

             circle square star
     blue         8     41   18
     orange       5     33   25
     red         53     64   58

    The rows and columns appear in essentially random order; they’re ordered by whatever order Python returns the dict keys during DataFrame initialization. Getting them in a specific order is left as an exercise for the reader.

    There’s one more detail to be aware of. If a particular (shape, color) combination doesn’t appear in my data, it will be represented by NaN in the DataFrame. They’re easy to set to 0 with frequencies.fillna(0).


    What I was trying to do with Pandas – unfortunately, the very first thing I ever tried to do with it – didn’t play to its strengths. It didn’t break my code, but it slowed it down by a factor of ~1700. Since I had thousands of items to process, the difference was hard to overlook!

    Pandas looks great for some things, and I expect I’ll continue using it. This was just a bump in the road, albeit an interesting one.

    Caktus GroupCaktus Attends Wagtail CMS Sprint in Reykjavik

    Caktus CEO Tobias McNulty and Sales Engineer David Ray recently had the opportunity to attend a development sprint for the Wagtail Content Management System (CMS) in Reykjavik, Iceland. The two-day software development sprint attracted 15 attendees hailing from a total of 5 countries across North America and Europe.

    Wagtail sprinters in Reykjavik

    Wagtail was originally built for the Royal College of Art by UK firm Torchbox and is now one of the fastest-growing open source CMSs available. Being longtime champions of the Django framework, we’re also thrilled that Wagtail is Django-based. This makes Wagtail a natural fit for content-heavy sites that might still benefit from the customization made possible through the CMS’ Django roots.

    Tobias & Tom in Reykjavik

    The team worked on a wide variety of projects, including caching optimizations, an improved content model, a new React-based page explorer, the integration of a new rich-text editor (Draft.js), performance enhancements, other new features, and bug fixes.

    David & Scot in Reykjavik

    Team Wagtail Bakery stole the show with a brand-new demo site that’s visually appealing and better demonstrates the level of customization afforded by the Wagtail CMS. The new demo site, which is still in development as of the time of this post, can be found at wagtail/bakerydemo on GitHub.

    Wagtail Bakery on laptop screen

    After the sprint was over, our hosts at Overcast Software were kind enough to take us on a personalized tour of the countryside around Reykjavik. We left Iceland with significant progress on a number of Wagtail pull requests, new friends, and a new appreciation for the country's magical landscapes.

    Wagtail sprinters on road trip, in front of waterfall

    We were thrilled to attend and are delighted to be a part of the growing Wagtail community. If you're interested in participating in the next Wagtail sprint, it is not far away. Wagtail Space is taking place in Arnhem, The Netherlands March 21st-25th and is being organized to accommodate both local and remote sprinters. We hope to connect with you then!

    Caktus GroupHow to write a bug report

    Here are some brief thoughts on writing good bug reports in general.

    Main elements

    There are four crucial elements when writing a bug report:

    • What did you do
    • What did you see
    • What did you expect to see
    • Why did you expect to see that

    What did you do

    This is sometimes called "Steps to reproduce".

    The purpose of this part is so the person trying to fix the bug can reproduce it. If they can't reproduce it, they probably can't fix it.

    The most common problem here is not enough detail.

    To help avoid that, it's a good idea to write this as though the person reading it knows nothing about the application or site that you ran into the problem on.

    Use words like "typed" and "clicked", not "I did such-and-such task".

    Say what you did, not what you meant. Use words like "typed" and "clicked", not "chose" or "selected" or "tried to".

    Good starting points: operating system name and version, browser name and version, what URLs you visited (exactly), what you typed and clicked. Pretend you're walking someone through what you did.


    I'm running Ubuntu 16.04.1 with Gnome desktop, using Chrome 54.0.2840.90 (64-bit).
    I recreated this in a incognito window (no extensions).
    I typed "https://www.example.com" into the address bar,
    Then I clicked the "Help" link in the top right.

    What did you see

    This is the obvious bit to include.

    Again, more detail is better. In particular, the exact wording of any messages - copy and paste if you can. A message that sounds generic to you might mean something very important to a developer trying to figure out the bug, if only they know the exact message you saw.

    Focus here on exactly what you saw, and not on interpreting it. If it's relevant, provide screenshots, labeling them to match the steps for reproducing the problem. If the problem is an observed behavior, still try to describe it in terms of what you saw in each step, and not how you interpret what you saw. Or provide a video of the bug happening.

    And if it doesn’t happen every time, be sure to say so, with a rough idea of how frequently you see it when you do the same things.


    After clicking "Help", a page loaded with URL
    "https://www.example.com/about" and the title "About WidgetCo".
    When I click the “Close” button, about one time in ten, the window doesn’t close.

    What did you expect to see

    This is often overlooked. It's surprising how often I get a report "the site did X" and my reaction is "well, the site is supposed to do X, what's the problem?" Then we have to go back and forth trying to figure out why the person submitted the bug report.

    It's much better to include this explicitly, even if it seems obvious to you.


    I expected to see a page with a title like "Help" or
    "Using this web site".

    Why did you expect to see that?

    This is the other often overlooked part.

    This can help save a lot of wasted time when it turns out that there's a typo in the documentation, or the user is missing some other part of the requirements, etc.

    A documentation error or unclear requirements are just as much bugs as broken code, but it's nice to zero in on what the problem is sooner than later.

    Did you expect to see "Y" because requirement 1.2.3 said it should happen? Was it on page N of version 2.1 of the user manual? Did the site help at URL xxxxx say something that led you to expect "Y"? Or maybe your officemate told you it worked that way.

    Another benefit is that in trying to find an authority on what was supposed to happen, you might discover that you misunderstood something and what you're seeing isn't a bug at all. Or you realize that what you thought was an authority really isn't.


    In my experience with other sites, links named "Help" go to
    pages with help information for using the site, not pages
    with information about the company.

    Other comments (optional)

    You can offer other information that you think would be helpful, but please do it separately from the previous elements - keep the facts separate from the opinions - and keep it concise.


    This looks to me like it's probably just the wrong link.
    Let me know if I can help test a fix or anything.
    My wife’s cousin’s girlfriend says it might be the frangistan coupling.

    Exceptions to the rules

    None of this is carved in stone. For example, if starting the application caused my laptop to hang so hard that I had to power it down, I can probably omit describing what I expected to see and why.

    More detail is generally better, but keep in mind that developers are human too, and probably won’t read the whole bug report carefully if it looks overly long.

    General tips

    Be very clear whether you are describing unexpected behavior, or asking for a change in behavior. Surprisingly, in some "bug reports", you can't really tell which the user means.

    Some phrases that should probably never appear in a bug report:

    • XXX didn't work
    • XXX doesn't work
    • XXX needs fixing
    • XXX should do YYY
    • XXX looks wrong

    Many applications and tools, and less often websites, have specific instructions on how they'd like bug reports to be submitted, what information is most helpful to include, etc. Look for and follow those instructions.

    Don't be emotional. If you're really annoyed about some behavior that's blocking your work, that's perfectly understandable. But it'll be more productive to take some time to cool down, then stick to the facts in your bug report.

    If you know that this behavior has changed - maybe this exact function worked for you in version 1.23 - then mention it in your comments. That kind of information is extremely helpful.

    If you haven't tried this on the most recent version of things, try it there. It might already be fixed.


    The most unique part of what I've described here is making sure to say why you expected what you expected. I know I saw that in a "how to write a good bug report" somewhere before, probably on the web, but it's been a long, long time. If anyone recognizes where that came from, please let me know in the comments.

    Meanwhile, here are some other pages that seems particularly good, and go into more detail about various of these points.

    Tim HopperHow I Quit My Ph.D. and Learned to Love Data Science

    I recently gave to the Duke Big Data Initiative entitled Dr. Hopper, or How I Quit My Ph.D. and Learned to Love Data Science. The talk was well received, and my slides seemed to resonate in the Twitter data science community.

    I've started a long-form blog post with the same message, but it's not done yet. In the mean time, I wanted to share the slides that want along with the talk.

    Philip SemanchukCoercing Objects to Integer, Revisited


    I recently wrote a blog post that involved exception handling, and gave short shrift to the part of exception handling I didn’t want to talk about in order to focus on the part I did want to talk about. For some readers, that clearly backfired.


    My recent blog post about coercing Python objects to integers caught people’s attention in a way I hadn’t intended. The point I was trying to make was that an innocent-looking call like int(an_object) calls the method an_object.__int__(), and since that can be arbitrary code, it can raise arbitrary exceptions. Therefore, it’s insufficient to catch only the usual exceptions of ValueError and TypeError if you don’t know the type of an_object in advance.

    Here’s the code I suggested –

    def int_or_else(value, else_value=None):
        """Given a value, returns the value as an int if possible.
        If not, returns else_value which defaults to None.
            return int(value)
        # I don't like catch-all excepts, but since objects can raise arbitrary
        # exceptions when executing __int__(), then any exception is
        # possible here, even if only TypeError and ValueError are
        # really likely.
        except Exception:
            return else_value

    Several commenters objected to the fact that this code discards (and therefore silences/masks/hides) all exceptions. Here’s why I made that choice.

    The Two Parts of Exception Handling

    In Python, there’s two parts to consider about exception handling — what to catch, and what to do with the exception once you’ve caught it. My intention was to write only about the former.

    The latter is an interesting topic, too. Once you’ve caught an exception, you might want to log it and then discard it, log it and then re-raise it, re-raise it as a different exception, silence it, let it pass up to the caller, modify its attributes and re-raise it, etc. There’s enough material for an entire blog post about different ways to react to an exception, and the pros and cons of each.

    Someday I might write that post about different ways to react to trapped exceptions, and if I do, I’ll dedicate the entire post to the subject to give it the attention it deserves. That other blog post – that was not it. In fact, it was the opposite. I gave the topic of processing the trapped exception as little attention as possible so as not to detract attention from what I wanted to be the main topic (what exceptions need to be trapped).

    That backfired.


    My post was not advocacy of discarding exceptions, nor was it advocacy of not discarding exceptions. What’s the right choice? It depends. One situation where you might want to discard exceptions is in a blog post where you’re trying to keep the code as brief as possible for readability. Then again, you might regret that. :-)

    In the future, I’ll be clearer about what shortcuts I’m taking for brevity of presentation.

    Agree? Disagree? I’d like to hear from you. I like it when people agree with me. Those who disagree can expand my horizons, and I like that too. In short, all civil comments are welcome. I feel I’ve spent enough time thinking about this topic for now, but that doesn’t make me right! Let me know what you think.

    Caktus GroupHow to make a jQuery

    Learn to live without jQuery by learning how to clone it

    jQuery is one of the earliest libraries every web developer learns, and often is the first experience with programming of any sort someone has. It provides a very safe cushion between a developer and the rough edges of web development. But, it can also obscure learning Javascript itself and learning what web APIs are capable of without the abstraction over them that jQuery adds.

    jQuery came about at a time when it was very much needed. Limitations in browsers and differences between them created enormous hardships for developers. These days, the landscape is very different, and everyone should consider withholding on adding jQuery to their projects until absolutely necessary. Forgoing it encourages you to learn the Javascript language on its own, not just as a tool to employ one massive library. You will be exposed to the native APIs of the web, to better understand the things you’re doing. This improved understanding gives you a chance to be more directly exposed to new and changing web standards.

    Let’s learn to recreate the most helpful parts of jQuery piece by piece. This exercise will help you learn what it actually does under the hood. When you know how those features work, you can find ways to avoid using jQuery for the most common cases.

    Selecting Elements on the Page

    The first and most prominent jQuery feature is selecting one or several elements from the page based on CSS selectors. When jQuery was first dropped on our laps, this was a mildly revolutionary ability to easily locate a piece of your page by a reliable, understandable address. In jQuery, selection looks like this:


    The power of the simple jQuery selection has since been adapted into a standard new pair of document methods: querySelector() and querySelectorAll(). These take CSS selectors like jQuery and give you the first or all matching elements in an array, but that array isn’t as powerful as a jQuery set, so let’s replicate what jQuery does by smartening up the results a bit.

    Simply wrapping querySelectorAll() is more than trivial. We'll call our little jQuery clone njq(), short for "Not jQuery" and use it the way you would use $().

    function njq(selector) {
        return document.querySelectorAll(selector)

    And now we can use njq() just like jQuery for selections.


    But, of course, jQuery gives us a lot more than this, so a simple wrapper won't do. To really match its power we need to add a few things:

    • Default to an empty set of elements
    • Wrap the original HTML element objects if we're given one
    • Wrap the results such that we can attach behaviors to them

    These simple additions give us a more robust example of what jQuery can do.

    var empty = $() // getting an empty set
    var html = $('<h2>test</h2>') // from an HTML snippet
    var wrapped = $(an_html_element) // wrapping an HTML Element object
    wrapped.hide() // using attached behaviors, in this case calling hide()

    So let's add these abilities. We'll implement the empty set version, wrapping Element objects, accepting arrays of elements, and attaching extra methods. We'll start by adding one of the most useful jQuery methods: the each() method used to loop over all the elements it holds.

    function njq(arg) {
        let results
        if (typeof arg === 'undefined') {
            results = []
        } else if (arg instanceof Element) {
            results = [arg]
        } else if (typeof arg === 'string') {
            // If the argument looks like HTML, parse it into a DOM fragment
            if (arg.startsWith('<')) {
                let fragment = document.createRange().createContextualFragment(arg)
                results = [fragment]
            } else {
                // Convert the NodeList from querySelectorAll into a proper Array
                results = Array.prototype.slice.call(document.querySelectorAll(arg))
        } else {
            // Assume an array-like argument and convert to an actual array
            results = Array.prototype.slice.call(arg)
        results.__proto__ = njq.methods
        return results
    njq.methods = {
        each: function(func) {
            Array.prototype.forEach.call(this, func)

    This is a good foundation, but jQuery selection has a few other required helpers we need to consider our version even close to complete. To be more complete, we have to add helpers for search both up and down the HTML tree from the elements in a result set.

    Walking down the tree is done with the find() method that selects within the children of the results. Here we learn a second form of querySelectorAll(), which is called on an individual element, not an entire document, and only selects within its children. Like so:

    var list = $('ul')
    var items = list.find('li')

    The only extra work we have left to do is to ensure we don't add any duplicates to the result set, by tracking which elements we've already added as we call querySelectorAll() on each element in the original elements and combine all their results together.

    njp.methods.find = function(selector) {
        var seen = new Set()
        var results = njq()
        this.each((el) => {
            Array.prototype.forEach.call(el.querySelectorAll(selector), (child) => {
                if (!seen.has(child)) {
        return results

    Now we can use find() in our own version:

    var list = njq('ul')
    var items = list.find('li')

    Searching down the HTML tree was useful and straight forward, but we aren't complete if we can't do it in the reverse: searching up the tree from the original selection. This is where we'll clone jQuery's closest() method.

    In jQuery, closest() helps when you already have an element, and want to find something up the tree in it. In this example, we find all the bold text in a page and then find what paragraph they're from:

    var paragraphs_with_bold = $('b').closest('p')

    Of course, multiple elements we have may have the same ancestors, so we need to handle duplicate results in this method, as we did before. We won't get much help from the DOM directly, so we walk up the chain of parent elements one at a time, looking for matches. The only help the DOM gives us here is Element.matches(selector), which tells us if a given element matches a CSS selector we're looking for. When we find matches we add them to our results. We stop searching immediately for each element's first match, because we're only looking for the "closest", after all.

    njq.methods.closest = function(selector) {
        var closest = new Set()
        this.each((el) => {
            let curEl = el
            while (curEl.parentElement && !curEl.parentElement.matches(selector)) {
                curEl = curEl.parentElement
            if (curEl.parentElement) {
        return njq(closest)

    We've put the basic pieces of selection in place now. We can query the page for elements, and we can query those results to drill down or walk up the HTML tree for related elements. All of this is useful, and we can walk over our results with the each() method we started with.

    var paragraphs_with_bold = njq('b').closest('p')

    Basic Manipulations

    We can't do very much with the results, yet, so let's add some of the first manipulation helpers everyone learned with jQuery: manipulating classes.

    Manipulating classes means you can turn a class on or off for a whole set of elements, changing its styles, and often hiding or showing entire bits of the page. Here are our simple class helpers: addClass() and removeClass() will add or remove a single class from all the elements in the result set, toggleClass() will add the class to all the elements that don't already have it, while removing it from all the elements which presently do have the class.

    The jQuery methods we're reimplementing work like this:


    Thankfully, the DOM's native APIs make all of these very simple. We'll use our existing each() method to walk over all the results, but manipulating the class in each of them is a simple call to methods on the elements' classList interface, a specialized array just for managing element classes.

    njq.methods.toggleClass = function(className) {
        this.each((el) => {
    njq.methods.addClass = function(className) {
        this.each((el) => {
    njq.methods.removeClass = function(className) {
        this.each((el) => {

    Now we have a very simple jQuery clone that can walk around the DOM tree and do basic manipulations of classes to change the styling. This, by itself, has enough parts to be useful, but some times just adding or removing classes isn't enough. Some times you need to manipulate styles and other properties directly, so we're going to add a few more small manipulation utilities:

    • We want to change the text in elements
    • We want to swap out entire HTML bodies of elements
    • We want to inspect and change attributes on elements
    • We want to inspect and change CSS styles on elements

    These are all simple operations with jQuery.


    Changing the contents of an element directly, whether text or HTML, is as simple as a single attribute we'll wrap with our helpers: text() and html(), wrapping the innerText and innerHTML properties, specifically. Like nearly all of our methods we're building on top of each() to apply these operations to the whole set.

    njq.methods.text = function(t) {
        this.each((el) => el.innerText = t)
    njq.methods.html = function(t) {
        this.each((el) => el.innerHTML = t)

    Now we'll start to get into methods that need to do multiple things. Setting the text or HTML is useful, but often reading it is useful, too. Many of our methods will follow this same pattern, so if a new value isn't provided, then instead we want to return the current value. Copying jQuery, when we read things we'll only read them from the first element in a set. If you need to read them from multiple elements, you can walk over them with each() to do that on your own.

    var msg_text = $('#message').text()

    These two methods are easily enhanced to add read versions:

    njq.methods.text = function(t) {
        if (arguments.length === 0) {
            return this[0].innerText
        } else {
            this.each((el) => el.innerText = t)
    njq.methods.html = function(t) {
        if (arguments.length === 0) {
            return this[0].innerHTML
        } else {
            this.each((el) => el.innerHTML = t)

    Next, all elements have attributes and styles and we want helpers to read and manipulate those in our result sets. In jQuery, these are the attr() and css() helpers, and that's what we'll replicate in our version. First, the attribute helper.

    $("img#greatphoto").attr("title", "Photo by Kelly Clark");

    Just like our text() and html() helpers, we read the value from the first element in our set, but set the new value for all of them.

    njq.methods.attr = function(name, value) {
        if (typeof value === 'undefined') {
            return this[0].getAttribute(name)
        } else {
            this.each((el) => el.setAttribute(name, value))

    Working with styles, we allow three different versions of the css() helper.

    First, we allow reading the CSS property from the first element. Easy.

    var fontSize = parseInt(njq('#message').css('font-size'))
    njq.methods.css = function(style) {
        if (typeof style === 'string') {
            return getComputedStyle(this[0])[style]

    Second, we change the value if we get a new value passed as a second argument.

    var fontSize = parseInt(njq('#message').css('font-size'))
    if (fontSize > 20) {
        njq('#message').css('font-size', '20px')
    njq.methods.css = function(style, value) {
        if (typeof style === 'string') {
            if (typeof value === 'undefined') {
                return getComputedStyle(this[0])[style]
            } else {
                this.each((el) => el.style[style] = value)

    Finally, because it's very common you want to change multiple CSS properties, and probably at the same time, the css() helper will accept a hash-object mapping property names to new property values and set them all at once:

        'background-color': 'navyblue',
        'color': 'white',
        'font-size: 40px',
    njq.methods.css = function(style, value) {
        if (typeof style === 'string') {
            if (typeof value === 'undefined') {
                return getComputedStyle(this[0])[style]
            } else {
                this.each((el) => el.style[style] = value)
        } else {
            this.each((el) => Object.assign(el.style, style))

    Our jQuery clone is really shaping up. With it, we've replicated all these things jQuery does for us:

    • Selecting elements across a page
    • Selecting either descendents or ancestors of elements
    • Toggling, adding, or removing classes across a set of elements
    • Reading and modifying the attributes an element has
    • Reading and modifying the CSS properties an element has
    • Reading and changing the text contents of an element
    • Reading and changing the HTML contents of an element

    That's a lot of helpful DOM manipulation! If we stopped here, this would already be useful.

    Of course, we're going to continue adding more features to our little jQuery clone. Eventually we'll add more ways to manipulate the HTML in the page, before we come back to manipulation let's start adding support for events to let a user interact with the page.

    Event Handling

    Events in Javascript can come from a lot of sources. The kinds of events we're interested in are user interface events. The first event you probably care about is the click event, but we'll handle it just like any other.

    $("#dataTable tbody tr").on("click", function() {
        console.log( $( this ).text() )

    Like some of our other helpers, we're wrapping what is now a standard facility in the APIs the web defines to interact with a page. We're wrapping addEventListener(), the standard DOM API available on all elements to bind a function to be called when an event happens on that element. For example, if you bind a function to the click event of an image, the function will be called.

    We might need some information about the event, so we're going to trigger our callback with this bound to the element you were listening to and we'll pass the Event object, which describes all about the event in question, as a parameter.

    njq.methods.on = function(event, cb) {
        this.each((el) => {
            // addEventListener will invoke our callback
            // with two parameters: the element the event
            // comes from and the event object itself.
            el.addEventListener(event, cb)

    This is a useful start, but events can do so much more. First, before we make our event listening more powerful, let's make sure we can hit the undo button by adding a way to remove them.

    var $test = njq("#test");
    function handler1() {
        $test.off("click", handler2)
    function handler2() {
    $test.on("click", handler1);
    $test.on("click", handler2);

    The standard addEventListener() comes paired with removeEventListener(), which we can use since our event binding was simple:

    njq.methods.off = function(event, cb) {
        this.each((el) => {
            el.removeEventListener(event, cb)

    Event Delegation

    When your page is changing through interactions it can be difficult to maintain event bindings on the right elements, especially when those elements could move around, be removed, or even replaced. Delegation is a very useful way to bind event handlers not to a specific element, but to to a query of elements that changes with the contents of your page.

    For example, you might want to let any <li> elements that get added to a list be removed when you click on them, but you want this to happen even when new items are added to the list after your event binding code ran.

    <h3>Grocery List</h3>
        <li>Peanut Butter</li>
    njq('ul').on('click', 'li', function(ev) {

    This very useful, but complicates our event binding a bit. Let's dive in to adding this feature.

    First, we have to accept on() being called with either 2 or 3 arguments, with the 3 argument version accepting a delegation selector as the second argument. We can use Javascript's special arguments variable to make this straight forward.

    njq.methods.on = function(event, cb) {
        let delegate, cb
        // When called with 2 args, accept 2nd arg as callback
        if (arguments.length === 2) {
            cb = arguments[1]
        // When called with 3 args, accept 2nd arg as delegate selector,
        // 3rd arg as callback
        } else {
            delegate = arguments[1]
            cb = arguments[2]
        this.each((el) => {
            el.addEventListener(event, cb)

    Our event handler is still being invoked for every instance of the event. In order to implement delegation properly, we want to block the handler when the event didn't come from the right child element matching the delegation selector.

    njq.methods.on = function(event, cb) {
        let delegate, cb
        // When called with 2 args, accept 2nd arg as callback
        if (arguments.length === 2) {
            cb = arguments[1]
        // When called with 3 args, accept 2nd arg as delegate selector,
        // 3rd arg as callback
        } else {
            delegate = arguments[1]
            cb = arguments[2]
        this.each((el) => {
            el.addEventListener(event, function(ev) {
                // If this was a delegate event binding,
                // skip the event if the event target is not inside
                // the delegate selection.
                if (typeof delegate !== 'undefined') {
                    if (!root.find(delegate).includes(ev.target)) {
                // Invoke the event handler with the event arguments
                cb.apply(this, arguments)
            }, cb, false)

    We've wrapped our event listener in a helper function, where we check the event target each time the event is triggered and only invoke our callback when it matches.

    Advanced Manipulations

    We have a good foundation now. We can find the elements we need in the structure of our page, modify properties of those elements like attributes and CSS styles, and respond to events from the user on the page.

    Now that we've got that in place, we could start making larger manipulations of the page. We could start adding new elements, moving them around, or cloning them. These advanced manipulations will the final set of helpers we add to our library.


    One of the most useful operations is adding a new element to the end another. You might use this to add a new <li> to the end of a list, or add a new paragraph of text to an existing page.

    There are a few ways we want to allow appending, and we'll add each one at a time.

    First, we'll allow simply appending some text.

    njq.methods.append = function(content) {
        if (typeof content === 'string') {
            this.each((el) => el.innerHTML += content)

    Then, we'll allow adding elements. These might come from queries our library has done on the page.

    njq.methods.append = function(content) {
        if (typeof content === 'string') {
            this.each((el) => el.innerHTML += content)
        } else if (content instanceof Element) {
            this.each((el) => el.appendChild(content.cloneNode(true)))

    Finally, to make it easier to select elements and append them somewhere else, we'll accept an array of elements in addition to just one. Remember, our njq query objects are themselves arrays of elements.

    njq.methods.append = function(content) {
        if (typeof content === 'string') {
            this.each((el) => el.innerHTML += content)
        } else if (content instanceof Element) {
            this.each((el) => el.appendChild(content.cloneNode(true)))
        } else if (content instanceof Array) {
            content.forEach((each) => this.append(each))


    As long as we are adding to the end of elements, we'll want to add to the beginning as well. This is a nearly identical to the append() version.

    njq.methods.prepend = function(content) {
        if (typeof content === 'string') {
            // We add the next text to the start of the element's inner HTML
            this.each((el) => el.innerHTML = content + el.innerHTML)
        } else if (content instanceof Element) {
            // We use insertBefore here instead of appendChild
            this.each((el) => el.parentNode.insertBefore(content.cloneNode(true), el))
        } else if (content instanceof Array) {
            content.forEach((each) => this.prepend(each))


    jQuery offers two replacement methods, which work in opposite directions.


    The first, replaceAll(), will use the elements from $('.left') and use those to replace everything from $('.right'). If you had this HTML:

    <h2>Some Uninteresting Header Text</h2>
    <p>A very important story to tell.</p>

    you could run this to replace the tag entirely, not just its contents:

    $('<h1>Some Exciting Header Text</h1>').replaceAll('h2')

    and your HTML would now look like this:

    <h1>Some Exciting Header Text</h1>
    <p>A very important story to tell.</p>

    The second, replaceWith(), does the opposite by using elements from $('.right') to replace everything in $('.left')

    $('h2').replaceWith('<h1>Some Exciting Header Text</h1>')

    So let's add these to "Not jQuery".

    njq.methods.replaceWith = function(replacement) {
        let $replacement = njq(replacement)
        let combinedHTML = []
        $replacement.each((el) => {
        let fragment = document.createRange().createContextualFragment(combinedHTML)
        this.each((el) => {
            el.parentNode.replaceChild(fragment, el)
    njq.methods.replaceAll = function(target) {

    Since this is a little more complex than some of our simpler methods, let's step through how it works. First, notice that we only really implemented one of them, and the second simply re-uses that with the parameters reversed. Now, both of these replacement methods will replace the target with all of the elements from the source, so the first thing we do is extract a combined HTML of all those.

    let $replacement = njq(replacement)
    let combinedHTML = []
    $replacement.each((el) => {
    let fragment = document.createRange().createContextualFragment(combinedHTML)

    Now we can replace all the target elements with this new fragment that contains our new content. To replace an element, we have to ask its parent to replace the correct child, using replaceChild():

    this.each((el) => {
        el.parentNode.replaceChild(fragment, el)


    The last yet easiest to implement helper is a clone() method. Allowing us to copy elements, and all their children, will make other helpers more powerful by allowing us to either move or copy them. This can be combined with other helpers we've already added, so that you have control over prepend and append operations moving or copying the elements they move around.

    njq.methods.clone = function() {
        return njq(Array.prototype.map.call(this, (el) => el.cloneNode(true)))

    Now You Made a jQuery

    We've replicated a lot of the things jQuery gives us out of the box. Our little jQuery clone is able to select elements on a page and relative to other elements via find() and closest(). Events can be handled with simple on(event, callback) bindings and more complex on(event, selector, callback) delegations. The contents, attributes, and styles of elements can be read and manipulated with text(), html(), attr(), and css(). We can even manipulate whole segments of the DOM tree with append(), prepend(), replaceAll() and replaceWith().

    jQuery certainly offers a much deeper and wider toolbox of goodies. We weren't aiming to create a call-for-call 100% compatible replacement, just to understand what happens under the hood. If you learned anything from this exercise, learn that the tools you use are all transparent and can be learned from. They're all layers on an onion that you can peel back and learn from.

    Philip SemanchukA Postcard of Tunisia

    Earlier on this blog I briefly mentioned working with some Libyans in Tunis, the capital of Tunisia. We chose to meet at that location because it’s close to Libya but much safer than Tripoli. Now that I’ve been back for a while and had a chance to catch up, I wanted to write more about my experience.

    A photo of the translator translating liveEnglish-to-Arabic translation on the fly!


    I was there with Tobias McNulty of Caktus Group. We (Tobias and I) trained the Libyan employees of Libya’s High National Election Commission (HNEC) in the maintenance and use of the HNEC-commissioned SMS-based voter registration system that I had helped to develop while working with Caktus. The system has been open sourced as Smart Elect.

    If the big picture was promoting democracy, the medium picture was training system admins and developers. And the very small picture was working together on the nitty gritty of features and bug fixes, like figuring out that if a @property method raises an exception when invoked by hasattr(), the exception isn’t propagated under Python 2.7.

    The admin training consisted of a comprehensive review of the system, including the obscure corners and edge case handling. The developers were eager to get their hands dirty, so after some organizational review, we dove into fixing bugs and implementing some new features that HNEC wanted.

    A photo of a traineeAbdullah (Photo by Tobias McNulty)

    Tobias and I worked with the developers as both mentors and peers. Grinding through bugs from start to finish was really valuable. Our trainees have good development experience, but working in groups with us allowed them to participate in our approach to debugging, problem reporting, development, and test. It seemed a little different from what they were used to. We were very methodical about creating an issue in our tracker, creating a branch for that issue, reviewing one another’s code, documenting the fix, etc. “It’s a lot of process,” said one trainee after working through one particular bug with us. He’s right. I wish I had thought to ask if Libyan culture has a proverb similar to “For want of a nail…“. I could have said, “For want of filing an issue in the tracker, a voter was disenfranchised,” but it doesn’t have the same ring to it.

    A photo of Tobias and a traineeTobias and Ahmed

    This was my first trip to Africa, and, grand notions aside, what stood out to me was how mundane much of the experience was. The guys we worked with would have fit right in at any coding meetup I’ve been to. They had opinions about laptops. They were distracted by their phones. Everyone enjoyed a successful bug hunt. I remember one trainee being tired at 5PM, saying he had no more left in him, and seeing him there grinning 2 hours later when we finally solved the problem we’d been working on.

    Outside of the training, I especially enjoyed the dinners at Sakura/Pasta Cosy and Chez Zina (my favorites, in that order).

    We also ate at Le Bon Vieux Temps, where the handwritten chalkboard menu is carted from table to table on a charming-but-impractical frame. Tunisia is principally French speaking, with Arabic on an almost equal footing. At Le Bon Vieux Temps (“The Good Old Times”), the menu was all in French, and my vestigial French came in handy for translating the menu into English for the Libyans who in turn peppered the waiter with questions in Arabic. (That night in the restaurant began and ended my career as a French-to-English translator.)

    On the weekends we rested, walked in the city, and paid a visit to the Bardo National Museum. The Bardo was famously attacked in 2015, and has since sprouted a razor wire fence around the entire property. Bored soldiers sat on a truck by the gate and motioned us to enter. It’s a nice museum, and I’m glad I went.

    A photo of my entrance pass to the Bardo Museum

    Inside the classroom and out, I got to know and really like our Libyan colleagues. They were generous with their good humor and kindness. If they lacked anything, it was a willingness to complain.

    Libya is a difficult place to live at the moment. I think we all know that in an abstract sense, but talking to my Libyan friends made it more concrete for me. Banks don’t have enough cash. Electricity isn’t reliable. People they know have been kidnapped. My friends have a lot on their minds, and yet they found rooom to squeeze in opinions about good software development practices.

    A photo of a traineeMunir

    I’m glad I got the chance to go, and to get to know the people I did. In addition to working with Tobias and the Libyans, I had a lot of non-work experiences I’ll remember for a long time. I walked among ruins in Carthage that are over 2000 years old. I drove solo (and lost) through rush hour traffic in Tunis and survived. I saw a Tunisian wedding, and got to use the word “ululating” for the first time outside of Scrabble or Bananagrams. I swam in the Mediterranean. I saw flocks of flamingoes (many, many thanks to Hichem and Claudia of Les Amis des Oiseaux).

    HNEC is now better positioned than ever to use the Smart Elect system, and I hope they do so again soon. That’s partly for egotistical reasons — I like to see my work get used. Who doesn’t? But more importantly, if it gets used, that means Libyans are voting to determine their own future.

    Caktus GroupCaktus at PyCaribbean

    For the first time, Caktus will be gold sponsors at PyCaribbean February 18-19th in Bayamon, Puerto Rico. We’re pleased to announce two speakers from our team.

    Erin Mullaney, Django developer, will give a talk on RapidPro, the open source SMS system backed by UNICEF. Kia Lam, UI Developer, will talk about how women can navigate the seas of the tech industry with a few guiding principles and new perspectives. Erin and Kia join fantastic speakers from organizations like 18F, the Python Software Foundation, IBM, and Red Hat.

    We hope you can join us, but if you can’t, there’ll be videos!

    Caktus GroupPlan for mistakes as a developer

    I Am Not Perfect.

    I've been programming professionally for 25 years, and the most important thing I have learned is this:

    • I am fallible.
    • I am very fallible.
    • In fact, I make mistakes all the time.

    I'm not unique in this. All humans are fallible.

    So, how do we still get our jobs done, knowing that we're likely to make mistakes in anything we try to do? We look for ways to compensate.

    Pilots use checklists, and have for decades. No matter how many times they've done a pre-flight check on a plane, they review their checklist to make sure they haven't missed anything, because they know it's important, people make mistakes, and the consequences of a mistake can be horrendous.

    The practice of medical care is moving in the same direction. There's a great book, The Checklist Manifesto by Atul Gawande, that I highly recommend if you haven't come across it before. It talks about the kind of mistakes that happen in medicine, and how adding checklists for even basic procedures had amazing results.

    I'm a big fan of checklists. I'm always pushing to get deploy and release processes, for example, nailed down in project documentation to help us make sure not to miss an important step.

    But my point is not just to use checklists, it's the reason behind the use of checklists: acknowledging that people make mistakes, and looking for ways to do things right regardless of that.

    For me, I try to find ways to do things that I'm less likely to get wrong now, and that make it harder for future me to screw them up. I know that future me will have forgotten a lot about the project by the time he looks at it again, maybe under pressure to fix a production bug.

    One of my favorite quotations about programming is by Brian Kernighan:

    Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it?

    ("The Elements of Programming Style", 2nd edition, chapter 2)

    So I work hard to avoid mistakes, both now and in the future.

    • I try to keep things straightforward
    • I use features and tools like strict typing, lint, flake8, eslint, etc.
    • I try to make sure knowledge is recorded somewhere more reliable than my memory

    I also try to detect mistakes before they can cause bad things to happen. I'm a huge fan of

    • unit tests
    • parameter checking
    • error handling
    • QA testing
    • code reviews

    To sum all this up:

    Expect to make mistakes. You will anyhow.

    Plan for them.

    And don't beat yourself up for it.

    Tim HopperYour Old Tweets from This Day

    A while ago, I published a Bash script that will open a Twitter search page to show your old tweets from this day of the year. I have enjoyed using it to see what I was thinking about in days gone by.

    So I turned this into a Twitter account.

    If you follow @your_old_tweets, it'll tweet a link at you each day that will show you your old tweets from the day. It attempts to send it in the morning (assuming you have your timezone set).

    This runs on Amazon Lambda. The code is here.

    Caktus GroupShip It Day Q1 2017

    Last Friday, Caktus set aside client projects for our regular quarterly ShipIt Day. From gerrymandered districts to RPython and meetup planning, the team started off 2017 with another great ShipIt.

    Books for the Caktus Library

    Liza uses Delicious Library to track books in the Caktus Library. However, the tracking of books isn't visible to the team, so Scott used the FTP export feature of Delicious Library to serve the content on our local network. Scott dockerized Caddy and deployed it to our local Dokku PaaS platform and serves it over HTTPS, allowing the team to see the status of the Caktus Library.

    Property-based testing with Hypothesis

    Vinod researched using property-based testing in Python. Traditionally it's more used with functional programming languages, but Hypothesis brings the concept to Python. He also learned about new Django features, including testing optimizations introduced with setupTestData.

    Caktus Wagtail Demo with Docker and AWS

    David looked into migrating a Heroku-based Wagtail deployment to a container-driven deployment using Amazon Web Services (AWS) and Docker. Utilizing Tobias' AWS Container Basics isolated Elastic Container Service stack, David created a Dockerfile for Wagtail and deployed it to AWS. Down the road, he'd like to more easily debug performance issues and integrate it with GitLab CI.

    Local Docker Development

    During Code for Durham Hack Nights, Victor noticed local development setup was a barrier of entry for new team members. To help mitigate this issue, he researched using Docker for local development with the Durham School Navigator project. In the end, he used Docker Compose to run a multi-container docker application with PostgreSQL, NGINX, and Django.

    Caktus Costa Rica

    Daryl, Nicole, and Sarah really like the idea of opening a branch Caktus office in Costa Rica and drafted a business plan to do so! Including everything from an executive summary, to operational and financial plans, the team researched what it would take to run a team from Playa Hermosa in Central America. Primary criteria included short distances to an airport, hospital, and of course, a beach. They even found an office with our name, the Cactus House. Relocation would be voluntary!

    Improving the GUI test runner: Cricket

    Charlotte M. likes to use Cricket to see test results in real time and have the ability to easily re-run specific tests, which is useful for quickly verifying fixes. However, she encountered a problem causing the application to crash sometimes when tests failed. So she investigated the problem and submitted a fix via a pull request back to the project. She also looked into adding coverage support.

    Color your own NC Congressional District

    Erin, Mark, Basia, Neil, and Dmitriy worked on an app that visualizes and teaches you about gerrymandered districts. The team ran a mini workshop to define goals and personas, and help the team prioritize the day's tasks by using agile user story mapping. The app provides background information on gerrymandering and uses data from NC State Board of Elections to illustrate how slight changes to districts can vastly impact the election of state representatives. The site uses D3 visualizations, which is an excellent utility for rendering GeoJSON geospatial data. In the future they hope to add features to compare districts and overlay demographic data.

    Releasing django_tinypng

    Dmitriy worked on testing and documenting django_tinypng, a simple Django library to allows optimization of images by using TinyPNG. He published the app to PyPI so it's easily installable via pip.

    Learning Django: The Django Girls Tutorial

    Gerald and Graham wanted to sharpen their Django skills by following the Django Girls Tutorial. Gerald learned a lot from the tutorial and enjoyed the format, including how it steps through blocks of code describing the syntax. He also learned about how the Django Admin is configured. Graham knew that following tutorials can sometimes be a rocky process, so he worked together with Graham so they could talk through problems together and Graham was able to learn by reviewing and helping.

    Planning a new meetup for Digital Project Management

    When Elizabeth first entered the Digital Project Management field several years ago, there were not a lot of resources available specifically for digital project managers. Most information was related to more traditional project management, or the PMP. She attended the 2nd Digital PM Summit with her friend Jillian, and loved the general tone of openness and knowledge sharing (they also met Daryl and Ben there!). The Summit was a wonderful resource. Elizabeth wanted to bring the spirit of the Summit back to the Triangle, so during Ship It Day, she started planning for a new meetup, including potential topics and meeting locations. One goal is to allow remote attendance through Google Hangouts, to encourage openness and sharing without having to commute across the Triangle. Elizabeth and Jillian hope to hold their first meetup in February.

    Kanban: Research + Talk

    Charlotte F. researched Kanban to prepare for a longer talk to illustrate how Kanban works in development and how it differs from Scrum. Originally designed by Toyota to improve manufacturing plants, Kanban focuses on visualizing workflows to help reveal and address bottlenecks. Picking the right tool for the job is important, and one is not necessarily better than the other, so Charlotte focused on outlining when to use one over the other.

    Identifying Code for Cleanup

    Calvin created redundant, a tool for identifying technical debt. Last ShipIt he was able to locate completely identical files, but he wanted to improve on that. Now the tool can identify functions that are almost the same and/or might be generalizable. It searches for patterns and generates a report of your codebase. He's looking for codebases to test it on!

    RPython Lisp Implementation, Revisited

    Jeff B. continued exploring how to create a Lisp implementation in RPython, the framework behind the PyPy project project. RPython is a restricted subset of the Python language. In addition to learning about RPython, he wanted to better understand how PyPy is capable of performance enhancements over CPython. Jeff also converted his parser to use Alex Gaynor's RPLY project.

    Streamlined Time Tracking

    At Caktus, time tracking is important, and we've used a variety of tools over the years. Currently we use Harvest, but it can be tedius to use when switching between projects a lot. Dan would like a tool to make this process more efficient. He looked into Project Hampster, but settled on building a new tool. His implementation makes it easy to switch between projects with a single click. It also allows users to sync daily entries to Harvest.

    Tim HopperTop Ten Favorite Photos of 2016

    I spent a lot of time with my camera in 2016. Here are some of the results.

    2016 Top Ten