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

If the maintainers agree that it's desirable to automatically deduce optionality, I'll implement it.

This is actually an intended behaviour. Moreover, the implicit optionality of arguments that default to `None` is deprecated and will be removed in one of the future versions. (Note that since typing module is still provisional, this will likely happen without any additional notice.)

This bug report can be closed.