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python中的多重继承实例讲解

Jun 06, 2016 am 11:32 AM
python multiple inheritance

python和C++一样,支持多继承。概念虽然容易,但是困难的工作是如果子类调用一个自身没有定义的属性,它是按照何种顺序去到父类寻找呢,尤其是众多父类中有多个都包含该同名属性。

对经典类和新式类来说,属性的查找顺序是不同的。现在我们分别看一下经典类和新式类两种不同的表现:

经典类:

代码如下:


#! /usr/bin/python
# -*- coding:utf-8 -*-

class P1():
    def foo(self):
        print 'p1-foo'

class P2():
    def foo(self):
        print 'p2-foo'
    def bar(self):
        print 'p2-bar'

class C1(P1,P2):
    pass

class C2(P1,P2):
    def bar(self):
        print 'C2-bar'

class D(C1,C2):
    pass


if __name__ =='__main__':
    d=D()
    d.foo()
    d.bar()

执行的结果:

代码如下:


p1-foo
p2-bar

将代码实例,画了一个图,方便理解:

从上面经典类的输出结果来看,

实例d调用foo()时,搜索顺序是 D => C1 => P1,

实例d调用bar()时,搜索顺序是 D => C1 => P1 => P2

总结:经典类的搜索方式是按照“从左至右,深度优先”的方式去查找属性。d先查找自身是否有foo方法,没有则查找最近的父类C1里是否有该方法,如果没有则继续向上查找,直到在P1中找到该方法,查找结束。

新式类:

代码如下:


#! /usr/bin/python
# -*- coding:utf-8 -*-

class P1(object):
    def foo(self):
        print 'p1-foo'
       
class P2(object):
    def foo(self):
        print 'p2-foo'
    def bar(self):
        print 'p2-bar'
       
class C1(P1,P2):
    pass
   
class C2(P1,P2):
    def bar(self):
        print 'C2-bar'
       
class D(C1,C2):
    pass
   

if __name__ =='__main__':
    print D.__mro__   #只有新式类有__mro__属性,告诉查找顺序是怎样的
    d=D()
    d.foo()
    d.bar()

执行的结果:

代码如下:


(, , , , , )

p1-foo
C2-bar

从上面新式类的输出结果来看,

实例d调用foo()时,搜索顺序是 D => C1 => C2 => P1

实例d调用bar()时,搜索顺序是 D => C1 => C2

总结:新式类的搜索方式是采用“广度优先”的方式去查找属性。

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