You can package simple python projects, but as soon as there are too many huge deoendecies that use CPython and whatnot, it becomes impossible to generate the nix derivation. I just use imperative python-venv + pip install on those.
(disclaimer: it's still rough but it does work)
https://github.com/NixOS/nixpkgs/tree/master/pkgs/servers/ho...
It's a PITA but unlike pip and conda it's 100% reliable.
The whole point of NixOS is to manage this and to get rid of those manual steps that are error prone.
Nix promises to solve exactly this problem... so it's not clear what the real benefit of Nix is.
EDIT: a rain of silent downvotes?
I guess the reason is because python packaging/tooling varies wildly between projects, and there are a lot of bindings.
BTW a colleague was setting up the python project on a non-nix machine, and also had problems with dependencies, and ultimately had to do some nasty workarounds (disabling deps/features). To me, it seems endemic.
Python is not trivially automatized with Nix.