uv-managed interpreters are not discovered on macOS: wrong store directory #109

This was discovered looking into wntrblm/nox#1139. Report from that investigation below.

🤖 AI text below 🤖

Problem

_propose_from_uv falls back to platformdirs.user_data_path("uv") / "python" when neither UV_PYTHON_INSTALL_DIR nor XDG_DATA_HOME is set. On macOS that is ~/Library/Application Support/uv/python, but uv intentionally follows the XDG convention on macOS and stores interpreters in ~/.local/share/uv/python:

$ uv python dir
/Users/me/.local/share/uv/python

So on macOS the uv store is silently invisible to discovery. The interpreters are only found via the ~/.local/bin shims on PATH (which users can disable, and which downstreams such as nox are moving to suppress: wntrblm/nox#1139).

Reproduction (macOS, python-discovery 1.5.0)

With cpython-3.8.20-macos-aarch64-none present in ~/.local/share/uv/python and ~/.local/bin removed from PATH:

import os
os.environ['PATH'] = ':'.join(p for p in os.environ['PATH'].split(':') if '.local/bin' not in p)
from python_discovery import get_interpreter
print(get_interpreter('3.8'))  # None

os.environ['UV_PYTHON_INSTALL_DIR'] = os.path.expanduser('~/.local/share/uv/python')
print(get_interpreter('3.8'))  # found in the uv store

Suggested fix

Replicate uv's directory logic (XDG-style ~/.local/share/uv/python on every non-Windows platform, including macOS) instead of platformdirs. Windows may need a check too — uv uses %APPDATA%\uv\python (Roaming), while platformdirs.user_data_path("uv") points at Local; I have only verified the macOS behavior.

Environment: macOS 15 (arm64), python-discovery 1.5.0, uv 0.9.x.