Prevent Poetry from using a different Python version?

[teutoburg]

Hi,

We're using Poetry as part of our CI test matrix with different Python versions (currently 3.10 - 3.12) and operating systems (ubuntu, macos, windows). One of the main goals with this is to consistently verify that our project works on all of those. Today we had a minor hickup, where someone accidentally bumped the requires-python in the pyproject.toml file to ">=3.11". This should have been caught by the CI test matrix combination using 3.10, which I would expect to fail to install the project. However, it almost went unnoticed because Poetry was acting "too smart" here, with the message:

The currently activated Python version 3.10.18 is not supported by the project (>=3.11, <3.13).
Trying to find and use a compatible version. 
Using python3.12 (3.12.3)

This is obviously not the intended purpose of the test! Is there any way of preventing Poetry to switch to a different Python version? Or perhaps in the workflow setup to remove the pre-existing one? In any case, I need a solution that works cross-platform, ideally without having to add conditional steps for e.g. Windows vs. non-Windows or whatever.

For reference, this is the relevant part of the workflow setup:

    steps:
      - uses: actions/checkout@v5

      - name: Install Poetry
        shell: bash
        run: pipx install poetry==2.1.4

      - name: Set up Python
        uses: actions/setup-python@v5
        with:
          python-version: ${{ matrix.python-version }}

      - name: Install dependencies with Poetry
        run: |
          poetry install --with test --all-extras

      - name: Run Pytest with Poetry
        run: poetry run pytest -m "not webtest" --cov --cov-report=xml

Originally, we had the "Set up Python" step before the Poetry one, but that recently led to problems when GitHub migrated the "macos-latest" label to macOS-15, where it would always end up using the default Python 3.13. Switching the steps at least solved that...

I realize this question might be a bit off-topic in being more about GitHub workflows rather than Poetry itself, but I've been battling this for the past few hours and any help would be greatly appreciated!

[radoering]

Poetry should fail if you run poetry env use 3.10 and Python 3.10 is not allowed. However, it looks like it does not. I would classify this as bug.

At the moment, you can probably just add a step that checks the version of the used Python (e.g. with poetry run python --version).