多重继承

采用多重继承首先，主要的类层次仍按照哺乳类和鸟类设计：

class Animal(object):
    pass

# 大类:
class Mammal(Animal):
    pass

class Bird(Animal):
    pass

# 各种动物:
class Dog(Mammal):
    pass

class Bat(Mammal):
    pass

class Parrot(Bird):
    pass

class Ostrich(Bird):
    pass

现在，我们要给动物再加上Runnable和Flyable的功能，只需要先定义好RunnableMixIn和FlyableMixIn的类：

class RunnableMixIn(object):
    def run(self):
        print('Running...')

class FlyableMinIn(object):
    def fly(self):
        print('Flying...')

## 多重继承
class Dog(Mammal, RunnableMixIn):
    pass

class Bat(Mammal, FlyableMixIn):
    pass