class A(object):
    def f(self): print 'A: doing f()'
    def g(self): print 'A: doing g()'

class B(object):
    def f(self): print 'B: doing f()'
    def g(self): print 'B: doing g()'


class C(object):
    def __init__(self):  self.obj = A()
    def toA(self):       self.obj = A()
    def toB(self):       self.obj = B()

    def __getattr__(self, name):
        return getattr(self.obj, name)