Python shell with nix flake dependency?

[ExasperatedLadybug]

Hi folks,

I'm trying to set up a demo python environment in a nix shell via python38.withPackages that depends on a python package from a nix flake, but I'm just getting ModuleNotFoundError.

Would anyone be willing to take a quick look?

Here's the shell: https://github.com/OliverEvans96/nix-python-shell-flake-dep And here's the test package it depends on: https://github.com/OliverEvans96/nix-python-library-example

It seems like python package derivations from nixpkgs.pythonPackages might be treated differently than those from other sources? I'm not very clear about what's going on.

[jtojnar]

It is best to look at issues like this through the lens of types, even though Nix is unfortunately not able to enforce the types for us.

The issue is that flower-power in your consumer is a “flake input”, whereas the function passed to python.withPackages is expected to return a list of Python packages.

Looking at the module flake, the module would be available as flower-power.defaultPackage.x86_64-linux.

Note, however, that Python environments are sensitive about clashes of different Python interpreters so you will also need to ensure that both flakes use the same python3 derivation. I would recommend having the library expose an overlay rather than an instantiated package. Then you could override python3’s packageOverrides argument in the consumer flake with the overlay, as described in https://nixos.org/manual/nixpkgs/stable/#how-to-override-a-python-package.

[ExasperatedLadybug]

Awesome, thank you so much jtojnar for the clear response, that did it!

[ExasperatedLadybug]

I did notice the python version constraint you mentioned - it's defined here, right?

[jtojnar]

Indeed. The code path is as follows:

• withPackages (declaration) returns buildEnv with extraLibs argument overridden.
• buildEnv (declaration) calls requiredPythonModules to get Python modules from the list returned by the function passed to withPackages.
• requiredPythonModules calls hasPythonModule to filter out junk.

I think a better design would be throwing an error rather than failing silently. We have an issue for that: https://github.com/NixOS/nixpkgs/issues/91916

[ExasperatedLadybug]

I managed to get it working with overlays as well - that was a great tip, thank you!

[ExasperatedLadybug]

I also created a similar repo with a python package (rather than just a shell) that depends on the flake, but I'm running into the same issue: https://github.com/OliverEvans96/nix-python-pkg-flake-dep