Overridden attribute should inherit the type

[jirkadanek]

from typing import List

class A(object):
    a = []  # type: List[int]

class B(A):
    a = []

This pattern of declaring an attribute in a superclass and then overriding the value with empty list or dict is fairly common in my codebase. Therefore, when mypy forces me to annotate all the places where override happens, it is fairly painful.

% mypy  ab.py
ab.py:7: error: Need type annotation for 'a' (hint: "a: List[<type>] = ...")

% mypy  --py2 ab.py
ab.py:7: error: Need type comment for 'a' (hint: "a = ...  # type: List[<type>]")

I am inclined to call the behavior a bug, I'd expect no error, and both A.a and B.a to be considered of type List[int] by mypy in the above code.

% mypy --version
mypy 0.720

Seems related to #3903

[ilevkivskyi]

Duplicate of #4547