spec/conformance: function with TypeVar default and default argument used in context. #2213

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1. randolf-scholz

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2. JelleZijlstra commented on Mar 8, 2026

   

   JelleZijlstra

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   The semantics of TypeVar defaults on generic functions are explicitly unspecified (https://typing.python.org/en/latest/spec/generics.html#function-defaults), so adding this case would require a spec change. Your use case seems reasonable though!

   I'd write the test case with assert_type(arg.get(), str | None).
3. randolf-scholz commented on Mar 8, 2026

   

   randolf-scholz

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   @JelleZijlstra

   I'd write the test case with assert_type(arg.get(), str | None).

   Well, actually all type checkers succeed for this case, the problem is that with the outer context str, checkers like mypy forget about the S <: None constraints when no explicit default is given, and happily present S=str as a solution.

   Without outer context, the default S=None is used instead.
4. randolf-scholz commented on Mar 8, 2026

   

   randolf-scholz

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   So the spec I am essentially arguing for here for is:

   1. If a function parameter is annotated with a type-variable with a default, then the default argument for that parameter must be assignable to that type var default.

      def get[T = object](default: T = object()) -> T: ...  # OK, object() is assignable to object
      def get[T = object](default: T = "123") -> T: ...  # Also OK, "123" is assignable to object
      def get[T = str](default: T = None) -> T: ...  # E: None is not assignable to str

   2. If for a parameter annotated with a TypeVar with default, no arguments are provided in a function call, then this should be treated as-if an argument of the default type were provided

      def get[T = None](default: T = None) -> T: ...
      
      get()  # identical inferenence as get(None)
      
      def get[T = object](default: T = None) -> T: ...
      
      get()  # identical inference as get( object() ), although real default is None

   In particular in my original example, both result: str = arg.get() and result: str = arg.get(default=None) should give identical inference.

   👍React with 👍5Reacted by Jelle Zijlstra, Neil Girdhar, Michael H, Thibaut Decombe and Zverev Konstantin
5. AlexWaygood

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