多态性

class dog(object):
    def speak(self):
        print("狗叫")

class cat(object):
    def speak(self):
        print("猫叫")

class Person(object):
    def speak(self):
        print("人说普通话")
def speak(obj):
    obj.speak()

p = Person()
speak(p)
c = cat()
speak(c)
d = dog()
speak(d)