Using Pyenv with NixOS

Recently I decided to switch from Ubuntu to NixOS. Do not ask me why, it was just for fun mostly. One of the main ideas behind NixOS is to separation of dependencies: each new package is installed into separate sandbox with own scope of dependencies. By design it should make system significantly more stable but sometimes there are problems. One of such problems I faced with pyenv – a tool for simplifying python versions management.

The simplest way to install pyenv in NixOS is to add the following lines to your /etc/nixos/configuration.nix:

  { config, pkgs, ... }:

  let
    unstableTarball = fetchTarball
      "https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz";
  in {
    ...

    nixpkgs.config = {
      allowUnfree = true;
      packageOverrides = pkgs:
        with pkgs; {
          unstable = import unstableTarball { config = config.nixpkgs.config; };
        };
    };

  environment.systemPackages = with pkgs; [
    pkgs.gcc
    pkgs.gnumake
    pkgs.zlib
    pkgs.libffi
    pkgs.readline
    pkgs.bzip2
    pkgs.openssl
    pkgs.ncurses
    unstable.pyenv
  ];

  }

Here we do the following:

1. Add a link to unstable channel of nix packages;
2. Defina an additional source and name it unstable
3. Add zlib, libffi, readline, bzip2, openssl, ncurses because we need them to build Python
4. Add unstable.pyenv to install the latest available version of Pyenv

After sudo nixos-rebuild switch you will be able to try pyenv install python3.10 but most probably you immediatly gen an error like this one:

zipimport.ZipImportError: can't decompress data; zlib not available

It may looks strange because we installed zlib but the root of this problem is in the main benefit of NixOS: zlib .h and .so files are installed into container and gcc cannot find them when trying to build Python.

Maybe it is not the right way but it worked for me. We need to spicefy additional libraries and search path for headers via environment variables. Because NixOS install libraries in paths that contains hashes there is no unified and constant place like /usr/lib that may be hardcoded. Because of this I added the following lines into my configuration.nix:

  environment.sessionVariables = {
    PYENV_ROOT="$HOME/.pyenv";
    # pyenv flags to be able to install Python
    CPPFLAGS="-I${pkgs.zlib.dev}/include -I${pkgs.libffi.dev}/include -I${pkgs.readline.dev}/include -I${pkgs.bzip2.dev}/include -I${pkgs.openssl.dev}/include";
    CXXFLAGS="-I${pkgs.zlib.dev}/include -I${pkgs.libffi.dev}/include -I${pkgs.readline.dev}/include -I${pkgs.bzip2.dev}/include -I${pkgs.openssl.dev}/include";
    CFLAGS="-I${pkgs.openssl.dev}/include";
    LDFLAGS="-L${pkgs.zlib.out}/lib -L${pkgs.libffi.out}/lib -L${pkgs.readline.out}/lib -L${pkgs.bzip2.out}/lib -L${pkgs.openssl.out}/lib";
    CONFIGURE_OPTS="-with-openssl=${pkgs.openssl.dev}";
    PYENV_VIRTUALENV_DISABLE_PROMPT="1";
  };

In this case we are spicifying all the paths and also directly point to OpenSSL installation. After that pyenv install start working correctly. I spent a lot of time trying to find a right way to specify this variables, so I hope my short post may safe this time for someone else!