gh-118647: Add defaults to typing.Generator and typing.AsyncGenerator

[JelleZijlstra]

• Issue: Add TypeVar defaults for Generator and AsyncGenerator #118647

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📚 Documentation preview 📚: https://cpython-previews--118648.org.readthedocs.build/

[Daverball]

It might be worth considering also updating ContextManager and AsyncContextManager to match what I did in python/typeshed#11048 for AbstractContextManager and AbstractAsyncContextManager, now that you've laid the necessary groundwork for defaults in _SpecialGenericAlias.

But we can also save it for a follow-up PR, if you'd prefer that. A change in typeshed doesn't make sense until we test 3.13 to the CI.

[JelleZijlstra]

@Daverball Possibly, but note that we're one day away from the feature freeze for Python 3.13. I can spend some time on it later today, but please open a separate issue first.

@AlexWaygood if you have time, could you review this PR that just adds defaults for Generator and AsyncGenerator?

[graingert]

typing.Generator and typing.AsyncGenerator are deprecated, why are they getting new features?

[mikeshardmind]

typing.Generator and typing.AsyncGenerator are deprecated, why are they getting new features?

I would hope this isn't a real blocker here. From a user standpoint, users are told collections.abc variants are meant to behave identically. (they are actually documented as a deprecated alias and not as a separate object)

Deprecated alias to collections.abc.Generator.

(https://docs.python.org/3/library/typing.html#typing.Generator)

If this deprecation results in them not being equivalent anymore, you've broken the documented idea of them being equivalent.