# How to enable custom source package installation in Binder

written by on 2021-07-10 | tags:

When I make tutorial material, I often choose to write a custom library in support of the tutorial. For example, for this JAX-centric tutorial repository that I made (dl-workshop), all exercise answers are written as Python functions that we can call in the notebook (and inspected at one's own pace). Doing so helps me ensure that I have one single source of truth for the answers and can write tests against them (if I desire so). By organizing the code answers into submodules, I also gain an organizational mapping from answers to tutorial sections - which is incredibly handy for navigating the library.

When building a Binder container using a configuration file, it's sometimes difficult to include the custom source library in the Binder build. I could, in theory, use the pip section of an environment.yml file to install the custom source as follows:

name: my_project
channels:
- conda-forge
dependencies:
- python=3.9
- ...
- pip:
- ...
- -e src/.


However, there are cases where we might not necessarily want the current version of the custom library. In those cases, we may instead prefer a canonical version that we can reference (like the one that lives on the master/main branch). Sticking the -e src/. in environment.yml thus restricts the flexibility that we might otherwise need.

Suppose this level of flexibility is needed. In that case, we need to match the flexibility with the appropriate composition of tooling. Writing a Dockerfile could be the way out. Thankfully the Binder team made a fit-for-purpose abstraction called the postBuild script. Essentially postBuild is nothing more than a shell script that gets executed right after the Docker container is built. We can use it to install our custom source library for use with Binder:

1. Include a file named postBuild in the root of our repository. It shouldn't have any file extensions, such as .sh or .zsh.
2. In there, add in the following commands, assuming our source code library is in the directory src/:
#!/bin/bash
set -e  # don't allow errors to fail silently
echo which pip  # in case the wrong pip is used, this will let us debug
pip install -e src/  # install the custom library


As a side note, Binder also provides start. This is a way of running code before the user session starts (e.g. setting environment variables that we don't want to be stored in a container). I have yet to find a use for this myself, but I'm sure the good folks on the Binder team have excellent reasons for doing so.

I really do have to give it to the Binder team. They've done a fantastic job here in architecting the package to satisfy workflow needs. Kudos!