[Python] How to tell pylint about type checks

The example below does a type check inside a function in order to keep the foo function cleaner. However,the last return values.a gets a static type check error. Pylint does not understand that values.a was already tested for None and it's actually safe to return values.a as a string.

class Values(TypedDict):
a: str | None
b: str | None

def raise_if_null(values: Values):
if(values.a is None):
raise ValueError("bla")
if(values.b is None):
raise ValueError("bla")


def foo(values: Values) -> str:
raise_if_null(values)

return values.a


How can I make pylint aware of the type check inside the function?

(This question must have already been answered somehow, I guess. I just cannot find the right search word for it)

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