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HomeBackend DevelopmentPython Tutorial浅析Python中的多重继承

继承是面向对象编程的一个重要的方式,因为通过继承,子类就可以扩展父类的功能。

回忆一下Animal类层次的设计,假设我们要实现以下4种动物:

  1.     Dog - 狗狗;
  2.     Bat - 蝙蝠;
  3.     Parrot - 鹦鹉;
  4.     Ostrich - 鸵鸟。

如果按照哺乳动物和鸟类归类,我们可以设计出这样的类的层次:

201542895326973.png (415×298)

但是如果按照“能跑”和“能飞”来归类,我们就应该设计出这样的类的层次:

201542895420016.png (417×298)

如果要把上面的两种分类都包含进来,我们就得设计更多的层次:

  •     哺乳类:能跑的哺乳类,能飞的哺乳类;
  •     鸟类:能跑的鸟类,能飞的鸟类。

这么一来,类的层次就复杂了:

201542895538159.png (415×385)

如果要再增加“宠物类”和“非宠物类”,这么搞下去,类的数量会呈指数增长,很明显这样设计是不行的。

正确的做法是采用多重继承。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Runnable和Flyable的类:

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

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

对于需要Runnable功能的动物,就多继承一个Runnable,例如Dog:

class Dog(Mammal, Runnable):
  pass

对于需要Flyable功能的动物,就多继承一个Flyable,例如Bat:

class Bat(Mammal, Flyable):
  pass

通过多重继承,一个子类就可以同时获得多个父类的所有功能。
Mixin

在设计类的继承关系时,通常,主线都是单一继承下来的,例如,Ostrich继承自Bird。但是,如果需要“混入”额外的功能,通过多重继承就可以实现,比如,让Ostrich除了继承自Bird外,再同时继承Runnable。这种设计通常称之为Mixin。

为了更好地看出继承关系,我们把Runnable和Flyable改为RunnableMixin和FlyableMixin。类似的,你还可以定义出肉食动物CarnivorousMixin和植食动物HerbivoresMixin,让某个动物同时拥有好几个Mixin:

class Dog(Mammal, RunnableMixin, CarnivorousMixin):
  pass

Mixin的目的就是给一个类增加多个功能,这样,在设计类的时候,我们优先考虑通过多重继承来组合多个Mixin的功能,而不是设计多层次的复杂的继承关系。

Python自带的很多库也使用了Mixin。举个例子,Python自带了TCPServer和UDPServer这两类网络服务,而要同时服务多个用户就必须使用多进程或多线程模型,这两种模型由ForkingMixin和ThreadingMixin提供。通过组合,我们就可以创造出合适的服务来。

比如,编写一个多进程模式的TCP服务,定义如下:

class MyTCPServer(TCPServer, ForkingMixin):
  pass

编写一个多线程模式的UDP服务,定义如下:

class MyUDPServer(UDPServer, ThreadingMixin):
  pass

如果你打算搞一个更先进的协程模型,可以编写一个CoroutineMixin:

class MyTCPServer(TCPServer, CoroutineMixin):
  pass

这样一来,我们不需要复杂而庞大的继承链,只要选择组合不同的类的功能,就可以快速构造出所需的子类。
小结

由于Python允许使用多重继承,因此,Mixin就是一种常见的设计。

只允许单一继承的语言(如Java)不能使用Mixin的设计。

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