class null(object):
    pass

    @classmethod
    def __bases__(cls):
        stack = [cls]
        while True:
            try:
                stack.append(stack[-1].__base__)
            except:
                break
            if stack[-1] is cls:
                break
        stack.pop(0)
        stack.pop()
        return tuple(stack)


class A(null):
    a = 'a'
    b = '?'

class B(null):
    a = '?'
    b = 'b'


def stat():
    global a,b,x
    print()
    print('a', a )
    print('a.a', a.a )
    print('a.b', a.b )
    print()
    print('b', b )
    print('b.a', b.a )
    print('b.b', b.b )

    print()
    print('x',x)
    print('x.a',x.a)
    print('x.b',x.b)

a = A()
b = B()
x = A()

stat()

print("="*60)
try:
    print( x.__class__.__base__ )
except : print('no.__base__')
try:
    if not isinstance( x.__class__.__bases__ , tuple):
        raise Exception('no.__bases__')
    print( x.__class__.__bases__ )
except :
    print('no.__bases__','fixing')
    print( x.__class__.__bases__() )

x.__class__ = B

stat()

x <__main__.A object at 0x7fa7a4371c50>
x.a a
x.b ?
============================================================
<class '__main__.null'>
(<class '__main__.null'>,)

x <__main__.B object at 0x7fa7a4371c50>
x.a ?
x.b b

x <A object at 7f7aa1b0d5a0>
x.a a
x.b ?
============================================================
<class 'null'>
no.__bases__

[...]
x <A object at 7f7aa1b0d5a0>  # <========= should be B
x.a a
x.b ?

    @classmethod
    def __bases__(cls):
        stack = [cls]
        while True:
            try:
                stack.append(stack[-1].__base__)
            except:
                break
            if stack[-1] is cls:
                break
        stack.pop(0)
        stack.pop()
        return tuple(stack)