[Python] Should mypy infer type from Union options?

In the following code fn_int will only be called with an int as an argument, yet mypy complains I might be passing it a str (viceversa for fn_str).

Is there something I'm missing here? Should/can I somehow narrow down the type before passing arguments to fn_int and fn_str?

from typing import Union


def fn_int(arg: int) -> None:
print(arg)


def fn_str(arg: str) -> None:
print(arg)


def fn(arg: Union[str, int]) -> None:

if arg in range(4):
fn_int(arg)
elif arg == "str":
fn_str(arg)
else:
raise ValueError


> mypy mypy_test.py
mypy_test.py:15: error: Argument 1 to "fn_int" has incompatible type "Union[str, int]"; expected "int" [arg-type]
mypy_test.py:17: error: Argument 1 to "fn_str" has incompatible type "Union[str, int]"; expected "str" [arg-type]

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