Superclass fields incorrectly assumed to exist

[marcoeilers]

Mypy seems to always assume that classes have all fields defined in their superclass' __init__ method, even if their own __init__ never calls the superclass constructor and those fields therefore don't exist.

class A:
    def __init__(self) -> None:
        self.a = 12


class B(A):
    def __init__(self) -> None:
        self.b = 14


def m() -> int:
    ab = B()
    return ab.a + ab.b  # accepted by mypy, but fails at runtime

[ilevkivskyi]

Thank you for reporting this! I think, it is an example of known behaviour: mypy does not check whether a variable was actually defined, it only checks that a variable was declared, see e.g. #2400

[JukkaL]

Closing as a duplicate of #2400.