Constrained TypeVars with Any default

[srittau]

Bug Report

In python/typeshed#13683 I'm trying to change a type var to the following:

_I = TypeVar("_I", int, bytes, float, bool, default=Any)  # possibly memoryview types

Unfortunately, mypy complains: error: TypeVar default must be one of the constraint types [misc]. While this is of course generally sound advice, a default of Any should always be permissible, as this allows memoryview to stand for any possible memoryview type. Without it, users are forced to write memoryview[Any] in every instance, which is unreasonable.

Your Environment

• Mypy version used: 1.18.2

[sterliakov]

I agree with your position, yet the spec seems to disallow that:

For constrained TypeVars, the default needs to be one of the constraints. A type checker should generate an error even if it is a subtype of one of the constraints.

[srittau]

Thanks, for pointing this out. I've opened a discussion at https://discuss.python.org/t/constrained-type-vars-with-any-default/104799.