Using pip with Python

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1

I’m trying to get a Python virtualenv in which I could install some packages. Now I know that’s not the recommended way of doing things with Nix, but virtualenvs are very useful when locally developing packages that are not yet packaged in Nix.

I need to use an old Python version (3.5), so I used the following shell.nix file, hoping it would allow me to create a virtualenv and install the requirements:

let
  oldPkgs = import (builtins.fetchGit {
      name = "python35-nixpkgs";                                                 
      url = "https://github.com/NixOS/nixpkgs/";                       
      ref = "refs/heads/nixos-20.03";                     
      rev = "0cf6a5035932023c324bfda301b89190801c3f30";                                           
  }) {};          
  pkgs = import (builtins.fetchGit {
    url = "/home/sephi/projects/nixpkgs";
    ref = "origin/release-20.09";
    rev = "e716ddfac4be879ffbae75c3914a538dd5d4d12e";
  }) { };                                                                 

  python =
    oldPkgs.python35.withPackages (ps: [ ps.pillow ps.isort ps.pyls-isort ]);
in pkgs.mkShell {
  buildInputs = [
    python
    pkgs.nodejs-10_x
  ];
}

nix-shell correctly drops me in a shell with Python 3.5 installed, and I can create a virtualenv using python -m venv /tmp/venv. But this virtualenv doesn’t have pip installed (even if I add ps.pip to my python35 package), and trying to install it using ensurepip doesn’t work because it tries to install pip in the nix store:

$ /tmp/venv/bin/python -m ensurepip
Requirement already satisfied: setuptools in /nix/store/w364k4rsaqwycb74wpldm7mxhppjdpqp-python3.5-setuptools-41.2.0/lib/python3.5/site-packages/setuptools-41.2.0-py3.5.egg
Collecting pip
Installing collected packages: pip
Exception:
Traceback (most recent call last):
...
OSError: [Errno 30] Read-only file system: '/nix/store/z4f5y7hgpkdq66312kmjfphv57v9qlh0-python3-3.5.7/lib/python3.5/site-packages/pip-9.0.1.dist-info'

Setting --root seems to work:

$ /tmp/venv/bin/python -m ensurepip --root /tmp/venv/
Requirement already satisfied: setuptools in /nix/store/w364k4rsaqwycb74wpldm7mxhppjdpqp-python3.5-setuptools-41.2.0/lib/python3.5/site-packages/setuptools-41.2.0-py3.5.egg
Collecting pip
Installing collected packages: pip
Successfully installed pip-9.0.1

But the package gets installed in /tmp/venv/nix/store/z4f5y7hgpkdq66312kmjfphv57v9qlh0-python3-3.5.7/lib/python3.5/site-packages/pip.

Does anyone know how I can get pip installed in the virtualenv, so I can try installing the Python requirements (which I suspect will fail, but I couldn’t even try that yet ^^)? Or do you have an idea how I could do it differently (without having to package each dependency of the requirements.txt file)? I know there are some *2nix tools out there, but the one I had the most success (or rather, the least failures) with doesn’t support editable requirements (-e git+https://...).