For python, how to use venvShellHook with numtide / devshell

[geekodour]

We can use venvShellHook with plain shell.nix with something like

let
  pkgs = import <nixpkgs> { };
  pyPackages = pkgs.python311Packages;
in pkgs.mkShell {
  name = "py";
  venvDir = "./.venv";
  buildInputs = [
    pyPackages.pip
    pyPackages.venvShellHook
    pkgs.ruff # linter
    pkgs.python311
  ];

  postShellHook = ''
    pip install poetry
    unset SOURCE_DATE_EPOCH
  '';
}

And with vanilla flake with something like:

 devShells.default = pkgs.mkShell {
        nativeBuildInputs = with pkgs; [pkgs.python311Packages.pip pkgs.python311Packages.venvShellHook];
        venvDir = "./.venv";
 };

But with numtide / devshell it seems like I cannot use venvShellHook and hence no way to have the sweet virtual env created for me as I cd into the directory.

[yajo]

Did you find a solution?

[geekodour]

@yajo my solution was not using numtide devshell but using flake devshell with flake as described here: https://fasterthanli.me/series/building-a-rust-service-with-nix/part-10

Here's a flake i use for one of my projects:

• It has unstable overlay (while some well respected members of the community mentioned that it's not the ideal way to use overlay for accessing unstable but works for me as of the moment)
• Additionally uses process-compose for setting up PG (remove if not needed)
• If we want to use something as shell hook, you must use postshellHook because shellHook will be used by venvShellHook and if you set shellHook manually it overrides it.
• Also has a ci shell in addition to the default shell, remove if not needed

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";
    nixpkgs-unstable.url = "github:nixos/nixpkgs/nixos-unstable";
    flake-parts = { url = "github:hercules-ci/flake-parts"; inputs.nixpkgs-lib.follows = "nixpkgs"; };
    process-compose-flake.url = "github:Platonic-Systems/process-compose-flake";
  };

  outputs = inputs@{ flake-parts, ... }:
    flake-parts.lib.mkFlake { inherit inputs; } {
      imports = [
        # https://community.flake.parts/process-compose-flake
        inputs.process-compose-flake.flakeModule
      ];
      systems = [ "x86_64-linux" "aarch64-linux" ];
      perSystem = { config, pkgs, system, ... }:
        let
          pyPackages = pkgs.python311Packages;
        in
        {
          _module.args.pkgs = import inputs.nixpkgs {
            inherit system;
            overlays = [
              (final: prev: {
                unstable = import inputs.nixpkgs-unstable {
                  system = final.system;
                  config.allowUnfree = true;
                };
              })
            ];
          };
          process-compose = {
            localpg = {
              settings.processes = {
                postgres-server = {
                  command = ''pg_ctl -o "-p $PGPORT -h $PGHOST" -D $PGDATA start'';
                  is_daemon = true;
                  shutdown = { command = "pg_ctl -D $PGDATA stop"; };
                };
              };
            };
          };

          devShells = {
            default = pkgs.mkShell {
              name = "custom-py-shell";
              venvDir = "./.venv";
              nativeBuildInputs = [ ];
              buildInputs = [
                pkgs.python311
                pyPackages.pip
                pyPackages.venvShellHook
                pkgs.poetry
                pkgs.just # adhoc commands
              ];
              packages = [
                # debugging
                #pkgs.memray
                #pyPackages.objgraph
                #pyPackages.pudb
                pyPackages.ipython
                pyPackages.isort
                pyPackages.mypy
                pkgs.ruff
                pkgs.unstable.duckdb
                # pg
                pkgs.postgresql_16 # for exts: withPackages (p: [ p.pg_uuidv7 ]))
              ];
              postShellHook = ''
                mkdir -p $PGDATA
                if [ -d $PGDATA ]; then
                  initdb -U postgres $PGDATA --auth=trust >/dev/null
                fi
                export LD_LIBRARY_PATH="${pkgs.lib.makeLibraryPath [
                  pkgs.stdenv.cc.cc
                ]}"
              '';
            };
            ci = pkgs.mkShell {
              name = "ci";
              venvDir = "./.venv";
              buildInputs = [
                pkgs.poetry
                pyPackages.venvShellHook
              ];
              packages = [
                pkgs.just
                pyPackages.isort
                pyPackages.mypy
                pyPackages.pytest
                pkgs.ruff
              ];
              postShellHook = ''
                export PROJECT_ROOT=$GITHUB_WORKSPACE
              '';
            };
          };
        };
    };
}

Hope it helps!