Windows: Use the Python Launcher to Select Python Version

[danyeaw]

In macOS and Linux, pipx allows you to set the Python version using the --python option. For example, pipx install --python python3.11 black. However, in Windows, all Python executables are named python.exe. So the only solution I have found to tell pipx the interpreter is by giving the full path like:

pipx install --python 'C:\\Users\\dan\\AppData\\Local\\Programs\\Python\\Python311\\python.exe' black

The standard way to select a Python version in Windows is with the Python Launcher. For example, py -3.11, will launch Python 3.11. It is possible to use the launcher with pipx with pipx install --python py black. However, I don't see a way to tell py which version of Python to use.

How would this feature be useful?
In Windows on GitHub Actions, pipx is installed by default with Python 3.9. I would like to run the setup-python action and then tell pipx to use this later Python version.

Describe the solution you'd like
pipx install --python 3.11 black would use py -3.11 on Windows and python3.11 in Linux/macOS. An alternative might be --python py311 which is the format tox uses.

Describe alternatives you've considered
Reinstall pipx for each Python version, or use the full path to the Python executable.

I would be glad to help implement this if there is agreement that this would be a nice enhancement.

[uranusjr]

Instead of magically mapping X.Y to pythonX.Y, I would prefer to make this fail without the launcher. Otherwise this seems reasonable. Contributions are welcomed.

[danyeaw]

Thanks @uranusjr for the feedback.

I was thinking that it might be nice to have common commands across platforms so that pipx would figure out the right interpreter command, but I understand wanting to avoid magical mapping.

Do you have a preference between --python 3.11 and --python py311?

[uranusjr]

Personally I feel 3.10 reads better.

[danyeaw]

I started working on this today. I think the order of looking up python should be:

1. --python is specified and then we use py with the version passed. Potentially should set the PY_PYTHON environmental variable rather than trying to pass a py -3.11 command around.
2. Use the venv Python if in a venv
3. Use py
4. Use system Python

2-4 are already part of the DEFAULT_PYTHON determination. For 1, we can override the interpreter in the main module, and put the tests and logic with the interpreter module.

It took me a little while to figure out the workflow, all tests were failing with nox on Windows, and I submitted #1000 to fix it. Also being able to use nox -s tests-3.11 -- -k test_interpreter which was in the contributing docs is important since the test suite takes so long to run.