# shell.nix
let
  # We pin to a specific nixpkgs commit for reproducibility.
  # Last updated: 2024-04-29. Check for new commits at https://status.nixos.org/.
  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/cf8cc1201be8bc71b7cbbbdaf349b22f4f99c7ae.tar.gz") {};
in pkgs.mkShell {
  packages = [
    (pkgs.python3.withPackages (python-pkgs: with python-pkgs; [
      # select Python packages here
      setuptools
      wxPython
      lxml
      reportlab
      pytest
    ]))
  ];
}

Output from attempting to run nix-shell:

unpacking 'https://github.com/NixOS/nixpkgs/archive/cf8cc1201be8bc71b7cbbbdaf349b22f4f99c7ae.tar.gz' into the Git cache...
error:
       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:37:12:
           36|
           37|   strict = derivationStrict drvAttrs;
             |            ^
           38|

       … while evaluating derivation 'nix-shell'
         whose name attribute is located at /nix/store/4ab6vrcph07w6ra79bc04fy8bbcmb9r0-source/pkgs/stdenv/generic/make-derivation.nix:331:7

       … while evaluating attribute 'nativeBuildInputs' of derivation 'nix-shell'
         at /nix/store/4ab6vrcph07w6ra79bc04fy8bbcmb9r0-source/pkgs/stdenv/generic/make-derivation.nix:375:7:
          374|       depsBuildBuild              = elemAt (elemAt dependencies 0) 0;
          375|       nativeBuildInputs           = elemAt (elemAt dependencies 0) 1;
             |       ^
          376|       depsBuildTarget             = elemAt (elemAt dependencies 0) 2;

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: undefined variable 'wxPython'
       at /home/edwinc/Documents/code/trelby/shell.nix:11:7:
           10|       setuptools
           11|       wxPython
             |       ^
           12|       lxml