setup-uv doesn't use latest Python 3.9 on macos

[vivodi]

The following config installs Python 3.9.6 instead of Python 3.9.13.

jobs:
  tests:
    runs-on: macos-latest
    steps:
    - name: Install uv
      uses: astral-sh/setup-uv@v5
      with:
        python-version: 3.9
        enable-cache: true

actions/setup-python uses Python 3.9.13 on macos-latest, while setup-uv uses Python 3.9.6.

Proof

My test failed because of this version problem:
Failed test using setup-uv uses Python 3.9.6.
Successful test using actions/setup-python uses 3.9.13.

Run astral-sh/setup-uv@v5
  with:
    cache-dependency-glob: uv.lock
    enable-cache: true
    python-version: [3](https://github.com/vivodi/Flexget/actions/runs/12494794277/job/34864837941#step:3:3).9
    version: latest
    github-token: ***
    prune-cache: true
    ignore-nothing-to-cache: false
Downloading uv from "https://github.com/astral-sh/uv/releases/download/0.5.11/uv-aarch6[4](https://github.com/vivodi/Flexget/actions/runs/12494794277/job/34864837941#step:3:4)-apple-darwin.tar.gz" ...
......
......
Run uv run pytest -n logical --dist loadgroup --junit-xml=test-results.xml
============================= test session starts ==============================
platform darwin -- Python 3.9.6, pytest-[8](https://github.com/vivodi/Flexget/actions/runs/12494794277/job/34864837941#step:5:9).3.4, pluggy-1.5.0

Run actions/setup-python@v5
  with:
    python-version: 3.9
    check-latest: false
    token: ***
    update-environment: true
    allow-prereleases: false
Installed versions
  Version 3.9 was not found in the local cache
  Version 3.9 is available for downloading
  Download from "https://github.com/actions/python-versions/releases/download/3.9.13-111836878[2](https://github.com/vivodi/Flexget/actions/runs/12494892248/job/34865041628#step:3:2)0/python-[3](https://github.com/vivodi/Flexget/actions/runs/12494892248/job/34865041628#step:3:3).9.13-darwin-arm64.tar.gz"

[eifinger]

The rootcause is that uv searches for python executables that satisfy the requested version requirements before downloading a version. On macos there is 3.9.6 under /Applications/Xcode_15.4.app/Contents/Developer/usr/bin/python3.

I'm currently not sure what the best fix for that is. We could enforce downloading the latest version but by that would remove the capability to use system managed versions which e.g. get installed by actions/setup-python. This would break existing functionality.

[Ravencentric]

Would python-preference help here?
E.g.,

- name: Install uv
      uses: astral-sh/setup-uv@v5
      with:
        python-version: 3.9
        python-preference: "only-managed"

[vivodi]

A good solution is to add the use-latest-python field, which you can refer to in the check-latest of actions/setup-python.

- name: Install uv
  uses: astral-sh/setup-uv@v5
  with:
    python-version: 3.9
    use-latest-python: true

[vivodi]

We could enforce downloading the latest version but by that would remove the capability to use system managed versions which e.g. get installed by actions/setup-python.

But the fact is that the current behavior of astral-sh/setup-uv is different from actions/setup-python. actions/setup-python uses Python 3.9.13, while astral-sh/setup-uv uses Python 3.9.6, and the latest is Python 3.9.21.

I think astral-sh/setup-uv should at least align with actions/setup-python.