callable default func

mypy_test.py

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing import List, Callable, TypeVar, Optional

T = TypeVar('T')


def test(func: Callable[[str], T]) -> List[T]:
    list(map(func, "1"))


print(test(str))  # OK
print(test(int))  # OK


def test1(func: Callable[[str], T]=str) -> List[T]:
    # error: Incompatible types in assignment (expression has type "str", variable has type Callable[[str], T])
    return list(map(func, "1"))

print(test1())  # OK
print(test1(str))  # OK
print(test1(int))  # OK


def _to_str(elem: str) -> str:
    return str(elem)


def test2(func: Callable[[str], T]=_to_str) -> List[T]:
    # error: Incompatible types in assignment (expression has type Callable[[str], str], variable has type Callable[[str], T])
    return list(map(func, "1"))


print(test2())  # OK
print(test2(str))  # OK
print(test2(int))  # OK


def test3(func: Callable[[str], str]=str) -> List[str]:
    return list(map(func, "1"))

print(test3())  # OK
print(test3(str))  # OK
print(test3(int))  # OK cause error