Get overridden functions of subclass

You can access the parent classes with cls.__bases__, find all attributes of the parents with dir, and access all the attributes of the class itself with vars:

def get_overridden_methods(cls):
    # collect all attributes inherited from parent classes
    parent_attrs = set()
    for base in cls.__bases__:
        parent_attrs.update(dir(base))

    # find all methods implemented in the class itself
    methods = {name for name, thing in vars(cls).items() if callable(thing)}

    # return the intersection of both
    return parent_attrs.intersection(methods)

>>> get_overridden_methods(B)
{'a2'}

You can make use of the __mro__ tuple, which holds the method resolution order.

For your example:

>>> B.__mro__
( <class '__main__.B'>, <class '__main__.A'>, <class 'object'>) 

So you could loop over that tuple and check if a B method is also in one of the other classes.