Bug: issubclass() doesn't narrow TypeVar type with Protocol #8556

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1. JukkaL commented on Mar 20, 2020

   

   JukkaL

   on Mar 20, 2020

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   The narrowing seems to work to a certain extent, but mypy loses the correspondence between the type of response_type and the type variable R. This may be too difficult to for mypy to handle, so a # type: ignore or a cast will likely be required in the first return statement.

   After the if statement, mypy could figure out that the type should be narrowed. However, again mypy doesn't have the notion of narrowed, dependent type variable, which would be required here.

   So this would be nice to support, but it seems too complicated to be worth implementing. I'm keeping this issue open, however, in case there are other reports of this issue, or if somebody comes up with a simple way of implementing this.
2. JukkaL

   added

   false-positivemypy gave an error on correct codemypy gave an error on correct code

   feature

   priority-2-low

   topic-type-variables

   on Mar 20, 2020
3. antonagestam commented on Mar 20, 2020

   

   antonagestam

   on Mar 20, 2020

   ContributorAuthor

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   @JukkaL Gotcha, thanks for elaborating!

   I think I'll manage to work around this by splitting the TypeVar into two and using an overload.

   from typing import Any
   from typing import Protocol
   from typing import runtime_checkable
   from typing import Type
   from typing import TypeVar
   from typing import Union
   from typing import overload
   
   T = TypeVar("T", bound=Any)
   
   
   @runtime_checkable
   class Parsable(Protocol):
       @classmethod
       def parse(cls: Type[T], data: Any) -> T:
           ...
   
   
   RP = TypeVar("RP", bound=Parsable)
   RB = TypeVar("RB", bound=str)
   # for other functions using the original typevar
   R = Union[RP, RB]
   
   
   @overload
   def parse_object(response_type: Type[RP], item: Any) -> RP: ...
   
   @overload
   def parse_object(response_type: Type[RB], item: Any) -> RB: ...
   
   def parse_object(response_type, item):
       if issubclass(response_type, Parsable):
           return response_type.parse(item)
       return response_type(item)
4. AlexWaygood

   added

   topic-type-narrowingConditional type narrowing / binderConditional type narrowing / binder

   on Mar 29, 2022
5. ilevkivskyi

   closed this as completedin 5a0fa55on Jun 3, 2025
6. ilevkivskyi

   closed this as completedin #19183on Jun 3, 2025
7. BobTheBuidler

   added a commit that references this issue on Jun 3, 2025

   Narrow type variable bounds in binder (python#19183)

   ...

   5a0fa55