Using Nix to Develop in Python

Python and virtualenv

Initially, working with Python and virtualenv in NixOS was not an intuitive endeavor. Admittedly, it would be remiss of me to not acknowledge NixOS + (your tool of choice) in any sentence can send you down a rabbit hole of “fun” and interesting challenges… 😂 What began as me setting out to automate my development environment with appropriate Python-esque packaging and versioning, evolved into a search for a Nix-ish solution that could do the trick automagically. This led to a journey of discoveries aided by Nix Discourse, NixOS wiki documentation, and StackOverflow suggestions.

Eventually, after extensive sifting through commentary and cursory read-throughs, I was able to craft together an approach. This led to what felt like a rather elegant and powerful solution. The result of my efforts enabled me to open a terminal in my dev environment, with everything loaded without any prompting whatsoever! If you are familiar with NixOS, then you are probably well acquainted with the functionality I am alluding to.

The Aproach

nix-shell in combination with nix-direnv allows for seamless loading of pre-configured shell environments without any needed clicks, manual running of local scripts, or one-off bash commands. Below I have included example code from my StackOverflow post.

nix-direnv Magic

For this flow, in the root directory of your project you will want the following file to enable nix-direnv:

if ! has nix_direnv_version || ! nix_direnv_version 3.0.4; then
  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/3.0.4/direnvrc" "sha256-DzlYZ33mWF/Gs8DDeyjr8mnVmQGx7ASYqA5WlxwvBG4="
fi
use_nix

Shell Configurations

Next define whatever configurations you need in a shell.nix file, for nix-direnv to load, like in the file below:

{ pkgs ? import <nixpkgs> { } }:
pkgs.mkShell {
  # nativeBuildInputs is usually what you want -- tools you need to run
  nativeBuildInputs = with pkgs.buildPackages; [
    python39Full
    python39Packages.virtualenv
  ];
  shellHook = ''
    virtualenv --python="${pkgs.python39Full}/bin/python" ./venv
    source ./venv/bin/activate
  '';
}

The Result

Finally, if you are using Nix and have the pre-requisite dependencies, when you open a terminal, vscode, etc, it should look something like the following:

~/test-py via 🐍 v3.9.18 (venv) via ❄️  impure (nix-shell-env) took 1m30s
❯

~/test-py via 🐍 v3.9.18 (venv) via ❄️  impure (nix-shell-env)
❯ ls -a
.  ..  .direnv  .envrc  shell.nix  venv

With code similar to the example above, I was able to quickly iterate from this point, adding needed packages and developing in Python with relative ease. Which ultimately allowed me to finish an MVP. While I am still learning a great deal when it comes to NixOS, working out the nuances was well worth the gains in efficiency and reproducibility of local shell environments.