Option to setup the oldest available Python rather than the latest version when a range of version specified

Description:
Describe your proposal.

When specifying the python-version with a version range or 3.x, the latest version will be installed.

runs-on: ubuntu-latest
steps:
  - name: Checkout
    uses: actions/checkout@v4

  - name: Set up Python
    uses: actions/setup-python@v5
    with:
      python-version: "3.7 - 3.13"

  - name: Lint
    run: pipx run pre-commit ...

This issue requests an opt-in option to let the action install the oldest major version available on the runner rather than the latest.

- name: Set up Python
  uses: actions/setup-python@v5
  with:
    python-version: "3.7 - 3.13"
    oldest-first: true

Justification:
Justification or a use case for your proposal.

Newer Python may introduce new syntax. The valid code that can run on the latest Python may contain syntax errors in older Python. For example, Python 3.12 supports more powerful f-strings.

$ cat test.py
lines = ['a', 'b', 'c']
string = f'The joined lines are:\n{"\n".join(lines)}'
print(string)

$ python3.12 test.py
The joined lines are:
a
b
c

$ python3.11 test.py
  File "/home/PanXuehai/.../test.py", line 2
    string = f'The joined lines are:\n{"\n".join(lines)}'
                                                         ^
SyntaxError: f-string expression part cannot include a backslash

The linting tool may not complain about the backward incompatible code if the linter runs with the latest Python.

One solution is to pin an old Python in the CI manually. However, the GitHub-hosted runner image may be updated separately with actions/setup-python. Users cannot use:

runs-on: ubuntu-latest
steps:
  - name: Set up Python
    uses: actions/setup-python@v5
    with:
      python-version: "3.7"

Because the CI will break when ubuntu-latest changes from ubuntu-22.04 to ubuntu-24.04.

- The version '3.7' with architecture 'x64' was not found for Ubuntu 24.04.
- The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

This request reduces the potential CI breakage for the two cases above.

Are you willing to submit a PR?

Inizia tracciando il modo in cui actions/setup-python risolve l’intervallo di python-version e consulta il comportamento del manifest delle versioni descritto nell’issue. Definisci il comportamento previsto per un input oldest-first con opt-in, incluse le modifiche all’immagine del runner, e verifica che il comportamento esistente di latest-version rimanga invariato.