ci: Introduce code formatter

🚀 Feature

We’ve spent quite some time on fixing code formatting in code reviews by our own hands, but this should be done automatically by a Python code formatter.

List of formatters

• black by Python Software Foundation (https://github.com/psf/black)
  • This blog post by DeepSource says black is the most popular one.
• yapf by Google (https://github.com/google/yapf)
• (autopep8, …)

Additional context

I have used black with isort (for import formatting) and flake8 (for pep8 check) in Optuna, and it worked really well, so +1 for black.

In PL, black was added in https://github.com/PyTorchLightning/pytorch-lightning/pull/1610, but it hasn’t been really used… We can start to employ the code formatter here, and if there seems no problem with it, we can enable it in PL later, too.

cc: @Borda @PyTorchLightning/bolts-contributors

[akihironitta]

@oke-aditya @briankosw Thank you both for sharing the useful information! So, as I read your comments, I believe we should do the followings.

To do

1. add black to pre-commit hooks
   • https://black.readthedocs.io/en/stable/version_control_integration.html
2. add black to GitHub Actions
   • https://black.readthedocs.io/en/stable/github_actions.html
3. update flake8 and isort configs to avoid conflicts with black
   • https://black.readthedocs.io/en/stable/compatible_configs.html
4. add the badge to readme
   • https://black.readthedocs.io/en/stable/show_your_style.html

Let’s get to work on this once we have positive comments from other core members :]

[Borda]

we can easily check formatting including trailing-whitespaces

I think that we can do this check with flake8 right? but yes, lest try black here :]

UPDATE: well just check the listed yapf and it looks better, seems to be more flexible and particular I like COALESCE_BRACKETS see: https://www.kevinpeters.net/auto-formatters-for-python