PEP 821: Support for unpacking TypedDicts in Callable type hints

[Daraan]

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@ Editors and Team - There has been an initial discussion which went off track. I am inclined to open up a new one, or should the old one be continued?

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📚 Documentation preview 📚: https://pep-previews--4764.org.readthedocs.build/

[hugovk]

Hello, PEPs need a sponsor before opening and assigning a PEP number. Please can you find a core dev sponsor via the discussion thread, and we can re-open then?

See https://peps.python.org/pep-0001/#submitting-a-pep

Thanks.

[JelleZijlstra]

I can sponsor this PEP. I think PEP 821 is taken by now but @Daraan if you renumber it to the latest available number, we can merge it.

[hugovk]

823 is next, please update the files to use this.

[hugovk]

Actually, 821 isn't used, please keep it :)

[hugovk]

Please also wrap to ~80 columns.

[Daraan]

Thank you very much @JelleZijlstra for the sponsorship! Thank you very much @hugovk for the first review. I've applied your suggestions and additional minor changes.

[hugovk]

@JelleZijlstra Please review and merge when you're happy with it :)

[JelleZijlstra]

Do we need to change something about how TypeVar substitution works? I haven't checked.

[JelleZijlstra]

I feel like the spec should mostly be that this works the same as PEP 692 (now in the typing spec at https://typing.python.org/en/latest/spec/callables.html#unpack-for-keyword-arguments ). The semantics should mostly follow from general rules; if we have to define a complex set of new rules for this feature, I feel we're doing it wrong.

[Daraan]

I feel the same way. Most should follow from the existing rules we already have.

For that reason I also removed the notion about inline TypedDicts in 59f6cb3, I think it follows naturally.

[JelleZijlstra]

Pedagogically, teachers might want to introduce this feature before Protocols.

[JelleZijlstra]

This should follow from general rules. If TD is a generic TypedDict, then Callable[Unpack[TD], R] means unpacking from TD[Any], and Callable[Unpack[TD[T]], R] means the callable itself is generic over T.

[Daraan]

Removed the sentence

[Daraan]

Thank very much for the feedback and review :)

Currently I think if we should also add the case Callable[[..., Unpack[TD], R]], i.e. abritary extra positional arguments.
If so why then not also an expension to Callable[[int, ..., Unpack[TD], R]].
In my brain it kind of conflicts with the current Concatenate semantics, or rather it raises the question why does one have to use Concatenate in one case and not the other.

[Daraan]

Removed the sentence

[Daraan]

I feel the same way. Most should follow from the existing rules we already have.

For that reason I also removed the notion about inline TypedDicts in 59f6cb3, I think it follows naturally.

[Daraan]

Note to self: Depending on the question of allowing multiple TypedDicts "may appear only once"