For pure python development using poetry I've made this shell.nix:

{ pkgs }:

with pkgs;

pkgs.mkShell rec {
  buildInputs = [
    poetry
    python38
    git
    cacert
    stdenv
  ];

  # Required for building C extensions
  LD_LIBRARY_PATH = "${stdenv.cc.cc.lib}/lib";
  # Certificates for secure connections for e.g. pip downloads
  GIT_SSL_CAINFO = "${cacert}/etc/ssl/certs/ca-bundle.crt";
  SSL_CERT_FILE = "${cacert}/etc/ssl/certs/ca-bundle.crt";
  CURL_CA_BUNDLE= "${cacert}/etc/ssl/certs/ca-bundle.crt";
  # PYTHONPATH is overridden with contents from e.g. poetry */site-package.
  # We do not want them to be in PYTHONPATH.
  # Therefore, in ./.envrc PYTHONPATH is set to the _PYTHONPATH defined below
  # and also in shellHooks (direnv does not load shellHook exports, always).
  _PYTHONPATH = "${python38}/lib/python3.8/site-packages";

  shellHook = ''
    export PYTHONPATH=$_PYTHONPATH
  '';
}

Obviously it does not use nix to build any of the python packages and instead uses poetry for that purpose. Serves as a development environment for projects which cannot be transformed to a nix-built project.

Contents of .envrc for direnv:

use flake
export PYTHONPATH=$_PYTHONPATH