Should cache of python-build-standalone downloads be invalidated?

[tpgillam]

Context: I was wondering whether astral-sh/python-build-standalone#421 might have helped with #6893 (answer: 'no'). This required me knowing which build of python-build-standalone I was using.

Current uv behaviour seems to be that the version of python-build-standalone isn't included anywhere obvious, and we don't automatically invalidate our cache of downloaded interpreters based on this version.

e.g. we can update to a newer uv and still get an old python build:

> uv --version
uv 0.5.13
> rm -r .venv/
> uv run python -VV
Using CPython 3.12.8
Creating virtual environment at: .venv
Installed 39 packages in 77ms
Python 3.12.8 (main, Dec  6 2024, 19:59:28) [Clang 18.1.8 ]

(Perhaps there's a better way (?) but the only way I could think to figure out what build of python-build-standalone we're using is by looking at the build time included in python -VV, so here December 6th 2024. )

From #7036 (comment), we need to explicitly remove or reinstall all downloaded versions to get the newer build:

> uv python uninstall --all
Searching for Python installations
Uninstalled 4 versions in 181ms
 - cpython-3.11.9-linux-x86_64-gnu
 - cpython-3.12.5-linux-x86_64-gnu
 - cpython-3.12.8-linux-x86_64-gnu
 - cpython-3.13.0-linux-x86_64-gnu
> uv run python -VV
warning: Ignoring existing virtual environment linked to non-existent Python interpreter: .venv/bin/python3 -> python
Using CPython 3.12.8
Removed virtual environment at: .venv
Creating virtual environment at: .venv
Installed 39 packages in 70ms
Python 3.12.8 (main, Dec 19 2024, 14:33:20) [Clang 18.1.8 ]

(NB python now build on December 19th)

Overall I find the existing behaviour a slightly opaque / unintuitive... but also it's a pretty niche problem! Nonetheless, maybe there's scope for a little improvement?

Some assorted ideas / thoughts:

• add documentation that we don't invalidate managed python downloads here: https://docs.astral.sh/uv/concepts/python-versions/#managed-python-distributions
• inclusion of python-build-standalone version in uv python list output
• sometimes (probably rarely!) useful to be able to specify the precise version of python-build-standalone?
• automatic switching of interpreter build version based on uv version... but this also feels a little unintuitive.
• if updating a managed python build, what should happen to existing venvs? I'm not too sure.