Nix language-specific dependencies

The following Nix shell declaration has a common bug:

let
  pkgs = import (builtins.fetchTarball {
    name = "nixos-unstable-2024-06-05";
    url = "https://github.com/nixos/nixpkgs/archive/57610d2f8f0937f39dbd72251e9614b1561942d8.tar.gz";
    sha256 = "0k8az8vmfdk1n8xlza252sqk0hm1hfc7g67adin6jxqaab2s34n9";
  }) { };
in
pkgs.mkShell {
  packages = [
    pkgs.bashInteractive
    pkgs.python3Packages.pytest
    pkgs.python310
  ];
}

Running python within the shell works, and running pytest works, but the two are not connected:

$ python -c "import pytest"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'pytest'

The fix is to explicitly include the Python packages with Python itself:

let
  pkgs = import (builtins.fetchTarball {
    name = "nixos-unstable-2024-06-05";
    url = "https://github.com/nixos/nixpkgs/archive/57610d2f8f0937f39dbd72251e9614b1561942d8.tar.gz";
    sha256 = "0k8az8vmfdk1n8xlza252sqk0hm1hfc7g67adin6jxqaab2s34n9";
  }) { };
in
pkgs.mkShell {
  packages = [
    pkgs.bashInteractive
    (pkgs.python310.withPackages (pythonPackages: [ pythonPackages.pytest ]))
  ];
}

Many interpreters support this pattern. Have a look for “withPackages” in the nixpkgs manual.