mypy doesn't understand that the function returns a string

I have a function that returns a str or a specified fallback value (str or None). When giving a str fallback, the function is guaranteed to return a str. Running mypy on this however gives an error.

MWE:

from typing import Any


def f(x: str, fallback: str | None) -> str | None:
    if x.lower() == "yes":
        return "okay"
    return fallback


f("no", fallback="").lower()

f.py:10: error: Item "None" of "str | None" has no attribute "lower"  [union-attr]

How can I tell mypy that it’s safe to call lower() on the return value of the function?

>Solution :

from typing import overload


@overload
def f(x: str, fallback: str) -> str:
    ...


@overload
def f(x: str, fallback: str | None = None) -> str | None:
    ...


def f(x: str, fallback: str | None = None) -> str | None:
    if x.lower() == "yes":
        return "okay"
    return fallback


f("no", fallback="").lower()
f("no").lower()

(type-)checks out:

• f("no", fallback="").lower() is fine (since the fallback is passed in and a string)
• f("no").lower() still says Item "None" of "str | None" has no attribute "lower"

Of course, if fallback is actually None, this will return a None, so you’ve broken the pinky-promise your types say.