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

Empieza ejecutando mypy sobre el reproductor de Python proporcionado y rastrea cómo afectan las dos sentencias if a la reducción de tipos. Identifica las pruebas relevantes de comprobación de tipos o el punto de entrada y, después, añade un caso de regresión para este flujo de control. Se considera terminado cuando mypy acepta foo1(b) sin un error arg-type y conserva las comprobaciones existentes.