typing.Annotated[int, ...] is subclassable #96771

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1. sobolevn

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   topic-typing

   on Sep 12, 2022
2. sobolevn commented on Sep 12, 2022

   

   sobolevn

   on Sep 12, 2022

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   Related: #96080
   This PR introduced explicit __mro_entries__ to _AnnotatedAlias.
3. sobolevn commented on Sep 12, 2022

   

   sobolevn

   on Sep 12, 2022

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   But, Annotated was subclassable even in 3.10.0:

   Python 3.10.0 (default, Nov  1 2021, 10:24:06) [Clang 11.0.0 (clang-1100.0.33.16)] on darwin
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import typing
   >>> class Some(typing.Annotated[int, 'meta']): ...
   ... 
   >>> Some.__mro__
   (<class '__main__.Some'>, <class 'int'>, <class 'object'>)
4. sobolevn commented on Sep 12, 2022

   

   sobolevn

   on Sep 12, 2022

   Last edited by sobolevn

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   Looks like there's a test for quite the opposite: https://github.com/python/cpython/blame/53a54b781d1f05f2d0b40ce88b3da92d5d23e9d2/Lib/test/test_typing.py#L6713-L6717

   It ensures that Annotated can be subclassed. Related: #27841
5. AlexWaygood commented on Sep 12, 2022

   

   AlexWaygood

   on Sep 12, 2022

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   I disagree that it is a bug that you can subclass Annotated[int, ...]. I think you're meant to be able to use Annotated[int, ...] at runtime at all sites where you could use int. Why should code like this be disallowed?

   SpecialInt: TypeAlias = Annotated[int, "very special"]
   class Foo(SpecialInt): ...

   This works fine, and I don't see why the cases should be treated differently:

   SpecialInt: TypeAlias = int
   class Foo(SpecialInt): ...
6. AlexWaygood commented on Sep 12, 2022

   

   AlexWaygood

   on Sep 12, 2022

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   In any case, it would surely be backwards-incompatible to change this behaviour now?
7. sobolevn commented on Sep 12, 2022

   

   sobolevn

   on Sep 12, 2022

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   Yes, changing this will be backward incompatible. Even type-checkers can understand this:

   from typing import Annotated
   
   class Some(Annotated[int, 'meta']):
       pass
   
   reveal_type(Some)
   # ex.py:6: note: Revealed type is "Overload(def (Union[builtins.str, builtins.bytes, array.array[Any], mmap.mmap, ctypes._CData, pickle.PickleBuffer, typing.SupportsInt, typing_extensions.SupportsIndex, _typeshed.SupportsTrunc] =) -> ex.Some, def (Union[builtins.str, builtins.bytes], base: typing_extensions.SupportsIndex) -> ex.Some)"

   So, it probably should stay as-is.

   👍React with 👍1Reacted by Alex Waygood
8. sobolevn

   closed this as completedon Sep 12, 2022