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PEP 695: typing.TypeVar should be subclassable #104662

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Closed as not planned
PEP 695: typing.TypeVar should be subclassable#104662
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JelleZijlstra
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topic-typingtype-bugAn unexpected behavior, bug, or error

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@JelleZijlstra
JelleZijlstra
opened on May 19, 2023

Previously typing.TypeVar could be subclassed, and typing_extensions actually does this. However, the new C implementation of TypeVar doesn't allow subclassing. For compatibility, let's restore the ability to subclass it.

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  1. JelleZijlstra
    added
    type-bugAn unexpected behavior, bug, or error
    topic-typing
    on May 19, 2023
  2. JelleZijlstra
    self-assigned this
    on May 19, 2023

  3. JelleZijlstra commented on May 19, 2023

    @JelleZijlstra
    JelleZijlstra
    on May 19, 2023
    MemberAuthor

    Looking more at the previous implementation, it wasn't actually subclassable normally, only if you passed the special _root=True argument to __init_subclass__:

    cpython/Lib/typing.py

    Line 397 in 702ce87

    def __init_subclass__(cls, /, *args, **kwds):

    That was an explicitly private API, so I don't think we need to preserve this behavior in C.

  4. JelleZijlstra commented on May 20, 2023

    @JelleZijlstra
    JelleZijlstra
    on May 20, 2023
    MemberAuthor

    Alternative solution in python/typing_extensions#162.

  5. JelleZijlstra
    closed this as not plannedon May 20, 2023
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