Ignore incompatible type error when variable is checked in if statement

I'm using mypy for type checking in my Python projects and I've come across a situation where I believe mypy could be improved.

Consider the following code:

def foo1(a: str): ...

def foo2(a: str | None, b: str | None):
    if not a and not b:
        return None
    if not a:
        return foo1(b)
    return None

In this case, mypy raises an error:

Argument 1 to "foo1" has incompatible type "str | None"; expected "str" [arg-type].

However, the function foo1 is only called when b
is not None, so in practice, there is no type error.

The logic of the function ensures that b cannot be None when foo1(b) is called.
This is because in the first if statement, we check if both a and b are None.
If they are not, we proceed to the next if statement where we check if a is None.
If a is None, then b must not be None because of the previous check.
Therefore, foo1(b) is only called when b is not None.

Versions

Python: 3.10.10
MyPy: 1.7.1

Commencez par exécuter mypy sur le reproducteur Python fourni et suivez l’effet des deux instructions if sur le narrowing des types. Identifiez les tests de vérification de types pertinents ou le point d’entrée, puis ajoutez un cas de régression pour ce flux de contrôle. C’est terminé lorsque mypy accepte foo1(b) sans erreur arg-type tout en préservant les vérifications existantes.