Static typing

[jorenham]

I noticed that threadpoolctl did not have static typing support, so I quicky wrote some (non-intrusive) stubs. The .pyi stub covers 100% of the public api, and (almost?) everything of the private API.

I went with stubs instead of inline annotations because I noticed that performance matters, and inline annotations tend to negatively impact that. Another reason is that I'm (very) used to writing stubs, so I'm pretty biased haha. Note that there some automated tools available that merge .pyi stubs into a .py as inline annotations.

I must admit that I'm not very familiar with the codebase, so there's a chance that I made a small mistake here and there. So please let me know if you see anything questionable. AFAIK it's also fine to commit to this branch yourself.

Mypy's stubtest tool (the state-of-the art for validating stubs) seems to like it. I added a stubtest job to the CI which will help prevent any regressions.

I wasn't able to get flit to include threadpoolctl.pyi in the wheel with the original "flat" project layout. So I unfortunately had to change it to a package-based layout. The sdist and wheel are building juist fine, and include the .pyi and the py.typed. The need for a py.typed is explained at https://typing.python.org/en/latest/spec/distributing.html#packaging-typed-libraries . Note that even if the .pyi annotations were to be inlined in the .py, then you'd still need the py.typed, because otherwise static type-checkers will simply ignore the annotations. And I don't think that it's possible to package a py.typed if you have a "flat" project layout, but I might be wrong.

[jorenham]

ready for review :)

[jorenham]

I moved the CLI logic into a new __main__.py module. Locally python -m threadpoolctl seems to be working again.

[ogrisel]

Hi, thanks for the PR, and sorry for the late feedback. Personally, I think I would have preferred inline annotations. What is the expected performance difference between the 2 options.

@jeremiedbb do you have a preference?

[jorenham]

What is the expected performance difference between the 2 options.

Somewhere between 50 and 200 ms startup time, mostly because of the additional imports.

[ogrisel]

Since those are all standard libraries imports, I suppose that it will not be noticeable for any Python program that already uses type hints otherwise.

No strong opinion either way.

[jorenham]

How about I open another PR with inline annotations so we can compare the two?

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Edit: #200