Not the greatest fan of python, but when I've got to run a python script, I do `nix-shell -p 'python3.withPackages (ps: [ps.requests])' --command 'python3 your-script.py'` Note that there is one argument to -p and one argument to --command -- both are quoted. The argument to -p is a nix expression that will provide a python3 command, referring to a python3 with the requests package. The argument to --command is a bash script that will run python3 with the argument "your-script.py" i.e. it will run your-script.py with the python3 that has the requests package.

I think there's ways you can autoderive a python3 with specific packages from python dependency files, but I can't help you there. I do find AI to be reasonably helpful for answering questions like this: it just might sometimes require a bit of help that you want to understand the answer rather than receive a perfect packaged shell.nix file.

Do you have to figure this out? Sure, it's nice and "pure" if everything is configured through Nix but there is something to be said about being pragmatic. Personally, I just enabled nix-ld[0] and use uv to install and handle my Python versions and package dependencies. Much, much easier.

[0]: https://mynixos.com/nixpkgs/option/programs.nix-ld.enable

Easier and largely compatible with the rest of the world. Solving problems with "If we all switched to NixOS..." is a non-starter in most organizations.

My rule of thumb: keep a strict separation between my projects (which change constantly) and my operating system (which I set up once and periodically update). Any hard nix dependency inside the project is a failure of abstraction IMO. Collaborating with people on other operating systems isn't optional!

In practice this means using language-specific package management (uv, cargo, etc) and ignoring the nix way.