Add version to github action (and rewrite the whole thing while at it)

[stefanfoulis]

Allows setting the exact black version use with the github action.
This is useful to be able to avoid needing to deal with black formatting changes introduced in new black versions in unrelated pull-requests.

[ichard26]

@stefanfoulis are you still working on this? No worries if you are no longer so, just checking in.

[stefanfoulis]

@ichard26 I made some changes to make the tests pass. I suppose we should also add some tests, but I don't know how to test github actions with multiple configurations.

[ichard26]

@stefanfoulis thanks! I'll take a look eventually (disclaimer: "eventually" may take a while!). For the testing, I usually just use a spare GitHub repository to test in (e.g. https://github.com/ichard26/testblackaction) but I've heard of https://github.com/nektos/act which allows you to run GHA workflows locally (can't say if it's any good since I haven't used it, but it looks promising at a few quick glances!).

[cooperlees]

Sorry about missing this. Lets rebase + fix the help message. I'm happy for this to have no change log since it does not break the main black program itself.

[ichard26]

FYI, I'll take over and push this PR to completion @stefanfoulis, thanks for the initial work!

[ichard26]

I've hacking on this and I just realized that since now this is a composite action, dependency clashes are possible between Black's dependencies and whatever is installed by the rest of the containing workflow.

While a composite action does mean this action can be used on any OS and doesn't require a hacky workaround (and I don't even know if that works), the lack of isolation might be an issue (dependencies clashes, requires the host to have Python 3.6+ as the python pip is attached to).

@cooperlees WDYT?

[cooperlees]

I've hacking on this and I just realized that since now this is a composite action, dependency clashes are possible between Black's dependencies and whatever is installed by the rest of the containing workflow.

Maybe make a venv for black so we can never have dependency clashes?

[ichard26]

Example runs:

• successful ones (including deprecated black_args option): https://github.com/ichard26/testblackaction/actions/runs/888879560
• failing one due to invalid version: https://github.com/ichard26/testblackaction/actions/runs/888903332

[cooperlees]

Nice work. Just some nit picks / suggestions for you to take or leave. Thanks for all the work here. Great move going to Python rather than using bash :D

[ichard26]

Thanks @JelleZijlstra and @cooperlees for the reviews, they are super appreciated :)

Hopefully this is ready to go now @cooperlees!

[ichard26]

Thank you @stefanfoulis so much! While I may have been the person to push this to completion, without your initial work, I don't think we would've ever worked on this feature (or atleast for a quite a while!). I'm so happy with the end result, much better usability now. If you have any cycles to spare, it would be great if you could provide some feedback from your experience contributing to psf/black, more details here: #2238.

Thank you once again!