bpo-41559: Documentation for PEP 612

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Please skip news, it shares the same news entry as GH-23702, thanks!

https://bugs.python.org/issue41559

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@gvanrossum. Sorry, I did the reverse of what I'd initially promised - I was in a writing mood :). Really no hurry to review it though, we have a long way till 3.10 beta.

This felt more difficult to write than what I did for PEP 585 and PEP 604 combined! Thinking of useful examples which were also understandable by users and copy-pastable/fully working as is wasn't easy. Maybe this is a sign that the typing module needs some refactoring work by the future Docs WG people.

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Hmm I wonder if we should reorder the PEP-sized what's new items while we're at it. Strange that the current order is PEP 563, 613, 604, 612.

[gvanrossum]

Thanks so much for this very thorough doc update! I have just a few nits.

Next, do you want to help fixing the support for this in mypy? There's some code, but it's not working right.

[gvanrossum]

You don't need to add this to the note, but privately I'm interested in an example where it goes wrong?

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'''This example is from the typing module, though it will behave the same with the builtin GenericAlias'''fromtypingimportParamSpec, TypeVar, GenericP=ParamSpec('P')
T=TypeVar('T')
classX(Generic[P]): ...
# Empty tuple, because T is nested inside that listprint(X[int, str, T].__parameters__)

[Fidget-Spinner]

Thanks so much for this very thorough doc update! I have just a few nits.

Next, do you want to help fixing the support for this in mypy? There's some code, but it's not working right.

You're welcome! Unfortunately I have no clue how mypy works internally, so I don't think I'm the right person for that right now.

[gvanrossum]

Everything else is fine now!

[gvanrossum]

Okay!

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Thank you for taking the time to review this prosaic chunk of text. Getting this approved is an awesome way to start the new year!

BTW, test_nntplib seems to be failing for the entire cpython repo right now. I'm just going to wait it out and see if it magically fixes itself, or if the people at https://bugs.python.org/issue42794 find some other way to solve it.