Incorrect type inference with isinstance()

Originally reported by: Maarten ter Huurne (BitBucket: mthuurne, GitHub: mthuurne)

When I run pylint on the following code fragment:

#!python

class ClassA:
    attrib_a = 1

class ClassB:
    pass

def someFunc(obj):
    if isinstance(obj, ClassB):
        obj = ClassB()
    return isinstance(obj, ClassA) and obj.attrib_a == 0

it reports:

E: 10,39: Instance of 'ClassB' has no 'attrib_a' member (no-member)

I think there are two things wrong here:

obj is only replaced by a ClassB instance if the original object was already of type ClassB, but objects of other types could be passed, so it's not valid to conclude that ClassB is the only possible type for obj
if isinstance(obj, ClassA) is true, it is certain that obj is of type ClassA, which should override any speculative inference

Bitbucket: https://bitbucket.org/logilab/astroid/issue/70

pylint-bot / astroid-unofficial

Incorrect type inference with isinstance() #70