Introduce _typeshed.WeakUnion #7437
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stdlib/_typeshed/__init__.pyi
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| @@ -10,6 +10,25 @@ from os import PathLike | |||
| from typing import AbstractSet, Any, Container, Generic, Iterable, Protocol, TypeVar, Union | |||
| from typing_extensions import Final, Literal, final | |||
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| WeakUnion = Any | |||
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what if we do this:
| WeakUnion = Any | |
| class WeakUnion(Any): | |
| def __class_getitem__(cls, args: Any) -> Any: ... |
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mypy doesn't (yet) have a generalised understanding of __class_getitem__. It only understands a class as being generic if the class inherits from typing.Generic. https://mypy-play.net/?mypy=latest&python=3.10&gist=1a3ed0af028c24c591ea1e63d80954f6
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That's too bad. Putting __getitem__ on the metaclass doesn't work either. This might be easier to fix than full PEP 646 support though.
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I have reservations about teaching mypy about __class_getitem__, as I think __class_getitem__ should properly be seen as an implementation detail of the typing framework. But I agree that it would be nice for mypy to have a stronger understanding of metaclass __getitem__.
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Can you give an example of what __getitem__ with a metaclass would look like? And is there any type type checker that currently handles that? I think we should open issues with mypy, pyright, pytype & pyre if they don't support that currently.
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using it in a return type doesn't work with pyright:
variable not allowed unless it is a type alias
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Hmm. I guess maybe that's part of the "Unknown" vs "Any" distinction pyright has? The even dumber hack is from _typeshed.does_not_exist import WeakUnion as WeakUnion # type: ignore
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That doesn't help pyright still treats such types it cannot import as Unknown.
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@erictraut, how hard would it be for you to implement special-casing in pyright for a WeakUnion/UnsafeUnion symbol imported from _typeshed? It wouldn't need to be the full implementation of this proposed feature just yet; it would just need to be a special form semantically identical to Any that could be provided with arbitrary subscriptions.
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I'd prefer not to do it that way. This approach looks very hacky to me, and I don't think it should go into typeshed this way.
If it's going to be a special form, then I think we should go through a PEP process and define it formally. If we're going to use PEP 646 as I suggested, then let's do that — although we'd need to wait for some minimal support in mypy first.
If this is just going to be an Any for now, then there's no benefit in the short term. I don't see the need to rush it. Let's pick a sound approach and get this right.
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By the way, if it's appropriate to bikeshed here, I have a big preference for a name with "Any" (or possible "Unsafe") in it. The principle being that it should be obvious to users when a type involves unsafety. So candidates include:
(my vote is currently still for AnyOf) |
I like |
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
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According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉 |
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The test failures indicate that this would be very difficult to do without adding special-casing to type checkers to account for this feature. As such, I'm going to close this for now, but we can maybe revisit it after further discussion in the python/typing repo or the typing-sig mailing list. |
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