Use py launcher to select Python interpreter

[danyeaw]

• I have added an entry to docs/changelog.md

Summary of changes

This PR closes #940 by adding support to use the py launcher to select the Python interpreter when the --python option is provided.

The feature is used like pipx install --python 3.10 cowsay to install using the Python 3.10 interpreter. The only way to do this right now is by passing in the full path to the interpreter.

I also did some refactoring in nearby code to leave the codebase in better shape than I found it.

Test plan

Tested by running

> pipx install --python 3.10 cowsay`
> pipx reinstall --python 3.11 cowsay`
> pipx uninstall cowsay`
> pipx run --python 3.9 cowsay hi`

pipx install --python 3.100 cowsay raises a FileNotFound error, which is similar to the behavior in Linux if you run pipx install --python python3.100 cowsay.

[uranusjr]

Some thoughts:

1. There’s an unofficial py launcher that works cross platform. While it is not a part of the official distribution, it may be a good idea to extend this feature to all platforms since it costs us nothing.
2. I wonder if we should unify the value format a bit. For example, if py is present in PATH and the user provides pythonX.Y, maybe we could automatically strip python and try py -X.Y? Not sure.
3. On the flip side, if the user gives X.Y and py is not found, maybe we can automatically try pythonX.Y in Python? I am less in favour of this one; seems a bit too magical.

[danyeaw]

Hi @uranusjr, thanks so much for the feedback! I haven't used py launcher in Linux before, but I can investigate adding support for it as well. Could that be part of a follow-up PR?

For the 2nd and 3rd points, I thought that your direction in the original issue was to avoid magic.

The nice thing about the 2nd option would be that a single command could be used across platforms, but no magic is fine as well.

[uranusjr]

The second point is mainly to maintain existing usages. We already support --python python.X.Y and it would be nice to make it automatically work on Windows.

The UNIX launcher can be found here: https://github.com/brettcannon/python-launcher
It’s mostly compatible with the Windows one, so maybe the best way to approach this is to simply drop the Windows check in this PR and investigate further later.

[danyeaw]

Hi team, is there anything else needed before we merge this?

[dukecat0]

Thanks!

[danyeaw]

Thanks team for your reviews and getting this merged, I think this will be a nice improvement to pipx!