python doc: update virtualenv section

[danbst]

• make it easy to switch python 2/3
• add glibcLocales dep (not sure it is still required after glibc: C.UTF-8 locales #54485
  but manual is also used by users of Nixpkgs versions <19.03)
• use mkShell
• use libffi, as it is required for Python3 cryptography (in my setup it worked with Python2 without this dep)
• set PYTHONPATH. Either this or source venv/bin/activate was missing.

Closes #46909

cc @FRidh @MelleB

[FRidh]

Considering this example is not a supported workflow it should never have been part of the manual.

[danbst]

@FRidh mmm, what are the problems here? I do advocate exactly this (wiki section https://nixos.wiki/wiki/Python#Emulating_virtualenv_with_nix-shell), and do use exactly this at company (I'm forced to, because I use NixOS).

But in case this isn't supported by you, then it's fine. I'll merge it before branchoff if there are no other objections.

[danbst]

@FRidh I've found your concerns at #21837 (review)

Now I understand that even if this example can work for some (me included), it is not easy to make it work in general case.

[Mic92]

It wouldn't be necessary after #54485 however for nixpkgs itself we also need support on macOS, which is also possible to add, but I currently have not the OS to implement it and test it. If you are only up to support linux, then you can already use it.

[Mic92]

I think it is OK to have it in the manual, if we mention the limitations of it,
i.e. mention the problems with GC. It is also possible to use some packages from the nix store (numpy, pyqt) if one using --system-site-packages. The GC problem can be also worked around by using a persistent GC root https://github.com/direnv/direnv/wiki/Nix#persistent-shell

I even think we still need this option because using nixpkgs python package sets limits one to the versions given by nixpkgs. These are likely to be not compatible with whatever a different project is working on. Also we https://github.com/garbas/pypi2nix it forces one to create your own list of overrides for every package
that needs manual intervention, which can become painful for more complex projects (also it would try to rebuild it everytime from the start after an error).
Also the nix workflow does not allow you to have editable dependencies, which are sometimes necessary if you want to fix stuff in other packages or just add a breakpoint to the source code.

[danbst]

Closing this, as virtualenv isn't officially supported for Nixpkgs. But still referenced this PR from https://nixos.wiki/wiki/Python#Emulating_virtualenv_with_nix-shell

[pmyjavec]

@danbst ,

Sorry to resurrect an old PR, but I love this! Thank you.

[Pablo1107]

Another sorry for resurrecting this PR, but the example giving in the wiki is (maybe?) wrong?

Shouldn't in pkgs.stdenv.mkShell { be in pkgs.mkShell {?

[Mic92]

Another sorry for resurrecting this PR, but the example giving in the wiki is (maybe?) wrong?

Shouldn't in pkgs.stdenv.mkShell { be in pkgs.mkShell {?

yes. The wiki allows logins via github, if you want to fix it.