Cannot have OpenCV binding for python 2 and python 3 active at the same time

[tolstikh]

Issue description

I would like to have python bindings accessible to Python 2 and Python3 at the same time.

I can make the bindings work in Python 2 and Python 3 individually, but not at the same time.

Steps to reproduce

expression:

with import <nixpkgs> {};
stdenv.mkDerivation rec {
  name = "env";
  env = buildEnv { name = name; paths = buildInputs; };
  buildInputs = [
    python27
    python27Packages.opencv3
    python36
    python36Packages.opencv3
  ];
}

then run

$ nix-shell
[nix-shell:~/env]$ python2 -c "import cv2"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named cv2
[nix-shell:~/env]$ python3 -c "import cv2"
[nix-shell:~/env]$

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 4.15.0-32-generic, Ubuntu, 16.04.5 LTS (Xenial Xerus)`
 - multi-user?: `no`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.0.4`
 - channels(user): `"nixpkgs-18.09pre147772.d1ae60cbad7"`
 - nixpkgs: `/home/user/.nix-defexpr/channels/nixpkgs`

[dotlambda]

Just as the manual tells you, you sould always use python.withPackages.