no error when value of type type (aka type[Any]) passed to function with generic with constraint, generic inferred as the first constraint #12300

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1. DetachHead

   added

   bugmypy got something wrongmypy got something wrong

   on Mar 7, 2022
2. DetachHead

   changed the title [-]no error when value of type `type` (aka `type[Any]` passed to function with generic with constraint, generic inferred as the first constraint[/-] [+]no error when value of type `type` (aka `type[Any]`) passed to function with generic with constraint, generic inferred as the first constraint[/+] on Mar 7, 2022
3. KotlinIsland commented on Mar 7, 2022

   

   KotlinIsland

   on Mar 7, 2022

   Last edited by KotlinIsland

   Contributor

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   I think this is three issues in one 😳

   1. mypy doesn't care about type without type parameter and hides Any errors (🐞) No error with disallow-any-generics and type #12301
   2. type[T] generics default to <nothing> when they are any

   from typing import _T
   
   def foo(t: type[_T]) -> _T: ...
   
   t: type
   reveal_type(foo(t)) # <nothing>

   3. Constrained TypeVars default to their first constraint when they are anyified example in OP, and:

   from typing import TypeVar
   
   T = TypeVar("T", int, str)
   
   def foo(cls: type[T]) -> T: ...
   
   a = []
   
   reveal_type(foo(a)) # int

   This seems quite broken.

   👍React with 👍2Reacted by DetachHead and Flávio Lisbôa
4. flisboac commented on Mar 9, 2022

   

   flisboac

   on Mar 9, 2022

   More actions

   Regarding item 2, PEP-483 says the following:

   If a generic type appears in a type annotation with a type variable omitted, it is assumed to be Any.

   So, it is a bug. Curiously, with Type the correct behaviour is shown:

   from typing import TypeVar, Any, Type
   
   T = TypeVar('T')
   
   t: Type
   
   def test(cls: Type[T]) -> T: ...
   
   reveal_type(t)  # note: Revealed type is "Type[Any]"
   
   reveal_type(test(t))  # note: Revealed type is "Any"

   👍React with 👍1Reacted by DetachHead
5. JelleZijlstra

   added

   topic-type-variables

   on Mar 19, 2022