Inherited, overridden attribute type reverted to the top parent class type

The issue type is probably quite bad, but example should quickly clear things up:

class A:
    attr = None


class B(A):
    attr: str  # type: ignore


class C1(B):
    attr = "test"


class C2(B):
    pass


class D(C2):
    attr = "test"

mypy test.py outputs:

test.py:18: error: Incompatible types in assignment (expression has type "str", base class "A" defined the type as "None")

where line 18 is the last line of above example

As to why I do attr: str # type: ignore - in a nutshell, in my case A class comes from external source and they botched type hints there so I try to fix them in B. Not sure if it should have worked for C1, but if it did I see it breaking in D as a bug due mypy not being consistent, but then again maybe there is a good reason for it?.

Tested with mypy 0.720

Inizia eseguendo l’esempio di ereditarietà fornito con mypy 0.720 e confronta le diagnosi per C1 e D. Traccia la risoluzione dei tipi per gli attributi ereditati e sovrascritti, quindi conferma che il comportamento risultante sia coerente con la semantica prevista descritta nell’issue.