Annotation Overridden Through Multilevel Inheritance

[WillAyd]

Assuming a class structure as follows:

class Base:
    attr = None  # type: Union[int, str]

class Derived1(Base):
    attr = 1

class Derived2(Derived1):
    attr = 'foo'

I was surprised to be getting the following error with Mypy:

error: Incompatible types in assignment (expression has type "str", base class "Derived1" defined the type as "int”)

Within the scope of Derived1 it makes sense to me that mypy may restrict the type to just int, but in the subsequent child class of Dervied2 I was surprised that the implicit annotation in Dervied1 took precedence over the explicit annotation provided in the Base class

I was getting this error with Python 3.7.2 on mypy master

[ilevkivskyi]

I may be equally (if not more) surprising if something like this would fail:

x: int = Derived1.attr  # if the type of attr would still be union, this would fail

There are some situation where keeping supertype context is desirable, but this is already discussed in #4547