Nix venv fails to detect python version

[asgeirn]

Describe the bug
I am creating a venv using nix-shell. Using layout python in .direnv yields the following errors:

direnv: loading ~/src/asgeirn/prices/.envrc
environment:850: python: command not found
direnv: Could not find python's version

If I manually activate the venv before cd'ing into the directory, it works as expected.

To Reproduce

1. nix-shell -p python39
2. python -m venv .venv
3. source .venv/bin/activate
4. pip install -r requirements.txt
5. echo layout python > .envrc
6. In a new shell, direnv allow

Expected behavior
The venv should activate, and which python3.9 should point to .venv/bin/python3.9

Environment

• OS: MacOS Sonoma 14.3.1 (23D60)
• Shell: zsh 5.9
• Direnv version 2.34.0
• nix-shell (Nix) 2.20.4

Additional context

ls -l .venv/bin
total 88
-rw-r--r--  1 asgeir  staff  8834 22 mar 11:27 Activate.ps1
-rw-r--r--  1 asgeir  staff  1914 22 mar 11:27 activate
-rw-r--r--  1 asgeir  staff   863 22 mar 11:27 activate.csh
-rw-r--r--  1 asgeir  staff  2003 22 mar 11:27 activate.fish
-rwxr-xr-x  1 asgeir  staff   249 22 mar 11:28 f2py
-rwxr-xr-x  1 asgeir  staff   266 22 mar 11:28 normalizer
-rwxr-xr-x  1 asgeir  staff   254 22 mar 11:27 pip
-rwxr-xr-x  1 asgeir  staff   254 22 mar 11:27 pip3
-rwxr-xr-x  1 asgeir  staff   254 22 mar 11:27 pip3.9
lrwxr-xr-x  1 asgeir  staff    69 22 mar 11:27 python -> /nix/store/p74zff3wk6mcrs0y8vr848yhj6nxic8h-python3-3.9.18/bin/python
lrwxr-xr-x  1 asgeir  staff     6 22 mar 11:27 python3 -> python
lrwxr-xr-x  1 asgeir  staff     6 22 mar 11:27 python3.9 -> python

[asgeirn]

This .envrc works, but is this the intended way of doing it?

source .venv/bin/activate
layout python

[steinybot]

Looks like it uses python to figure out the python version and virtual environment to use:

direnv/stdlib.sh

Line 850 in 6a73cc2

read -r python_version ve <<<$($python -c "import importlib.util as u, platform as p;ve='venv' if u.find_spec('venv') else ('virtualenv' if u.find_spec('virtualenv') else '');print('.'.join(p.python_version_tuple()[:2])+' '+ve)")

[rybaz]

Ended up here because I had the same issue - is there a way to determine the python version and include it as part of the direnv instead of having to install it on the system prior? I don't really want to have python installed system-wide if I don't have to.