nix-shell expression with custom python packages

shell.nix

{ pkgs ? import <nixpkgs> {} }:

let
  inherit (pkgs) stdenv;
  pythonPackages = stdenv.lib.fix' (self: with self; pkgs.python2Packages //
  {

    atari-py = buildPythonPackage rec {
      name = "atari-py-${version}";
      version = "0.0.21";

      buildInputs = [ nose2 pkgs.cmake pkgs.zlib ];
      propagatedBuildInputs = [ numpy six ];

      src = pkgs.fetchurl {
        url = "https://pypi.python.org/packages/46/0b/96800c0d66ef36c774f8b2cc4589674352386b943e6603762d9b8167af99/${name}.tar.gz";
        sha256 = "0j5dl2h907gv7kf4r8smhxyk42vnpfifq212bnw0ncdg01x37njl";
      };
    };

    gym = buildPythonPackage rec {
      name = "gym-${version}";
      version = "0.7.0";

      src = pkgs.fetchurl {
        url = "https://pypi.python.org/packages/fb/0c/bc3e46d60b6f407ec591a2c2c1c6552407c56b7785eda9932a4152ec3b0b/${name}.tar.gz";
        sha256 = "0ghc938jpbbxyhn5szhc30byp00dkk51cqszcz64k523kahb7m7h";
      };

      # buildInputs = [ mock pytest ];
      propagatedBuildInputs = [ numpy requests2 six pyglet atari-py pillow pyopengl ];

      doCheck = false;
    };
  });
in

stdenv.mkDerivation {
  name = "user-environment";
  buildInputs = [ pythonPackages.gym pythonPackages.tensorflow pythonPackages.tqdm pythonPackages.opencv ];
}