class Foo(object):
    a = None
    b = None

    def __init__(self, b=None):
        self.a = 'A'
        self.b = b

    def return_ab(self):
        return self.a, self.b

    def return_child(self):
        return Bar()


class Bar(Foo):

    def example_method(self):
        print self.return_ab()

foo = Foo('B')
print foo.a  # A
print foo.b  # B
print foo.return_ab() # ('A', 'B')

foo.return_child().example_method()
#  ('A', None)

# Why does B not have a value when called from the child class, and how can this be rectified?