Default value of Type[T]-argument

[galcik]

from typing import Type, TypeVar

T = TypeVar("T", bound="Base")


class Base(object):
    pass


class Subclass(Base):
    pass


def create_instance(instance_type: Type[T] = Base) -> T:
    return instance_type()

Mypy (0.641) fails with:

Incompatible default for argument "instance_type" (default has type "Type[Base]", argument has type "Type[T]")

Revealed types of return values:

create_instance()  # <nothing>, but I would expect Base*
create_instance(Base)  # Base*, OK
create_instance(Subclass)  # Subclass*, OK

[ilevkivskyi]

@galcik All behaviour you see is expected. What you want is lower bound, while mypy only supports upper bounds for type variables, see discussions at the end of #3737, which I think this issue is a duplicate of.