from typing import *

def f(arg: str = None):
    pass
print(get_type_hints(f))

# {'arg': typing.Union[str, NoneType]}
# as expected


class T(NamedTuple):
    field: str = None
print(get_type_hints(T))

# {'field': <class 'str'>}
# but it should be
# {'field': typing.Union[str, NoneType]}
# for consistency