python 多继承分析

多继承在针对派生类中的同名函数时，python 需要对其进行线性化(C3 Linearization),将继承图关系线性化。通过线性化，再依次查找类方法，直至找到该方法为止。线性化算法是python多继承的核心部分。线性化过程中，必须满足两个性质:

单调性

如果C从C^'直接继承属性P(如图1),(C != C^'),C^'所有的父类(包含C^')均在线性化中都是C的子列表。这样的话，C的线性化列表不用遍历C的所有继承体系，直接通过C的直接继承父类线性化列表获得(通过归并父类列表获得)。

一致性

直接父类的顺序通过用户来声明，父类线性化的顺序为从左到右。在进行线性化归并过程中，一致性主要保证类的局部优先级顺序，它定义了两个变量:
1) precC:类C的局部顺序
2) prec:局部顺序的并集

类C1,C2,C1 < precC C2 ===> C1 < prec C2,prec 为无环关系图。

通过上面的两个定义，引申出最终边的关系图。扩展关系 a

假设类的继承关系图为:

通过计算扩展关系 5 2 4 4

这样最终的关系图为:

线性化关系为:

L(7) = {'object'}
L(4) = {'4','7'}
L(5) = {'5','7'}
L(6) = {'6','7'}
L(2) = {'2','4','5','7','object'}
L(3) = {'3','4','6','7','object'}
L(1) = {'1','2','3','4','5','6','7','object'}

python 代码如下:

#! /usr/bin/env python
#
# multi inheritance view
#                         /------ 7-------\
#                        /        |        \
#                       4       - 5         6
#                         \\   /           /
#                           2-/\--- 3------/
#                            \   /
#                             0,1
#
#

class C7(object):
    def test(self):
        print 'test in C7'

class C4(C7):
    def test(self):
        print 'test in C4'

class C5(C7):
    def test(self):
        print 'test in C5'

class C6(C7):
    def test(self):
        print 'test in C6'

class C2(C4,C5):
    def test(self):
        print 'test in C2'

class C3(C4,C6):
    pass

class C1(C2,C3):
    pass

class C0(C2,C3):
    def test(self):
        print 'test in C0'
        super(C3,self).test()

if __name__ == "__main__":

    c1 = C1()
    c1.test()
    c0 = C0()
    c0.test()
    print C7.__mro__
    print C6.__mro__
    print C5.__mro__
    print C4.__mro__
    print C3.__mro__
    print C2.__mro__
    print C1.__mro__
    print C0.__mro__

输出结果为:

test in C2
test in C0
test in C4
(<class '__main__.C7'>, <type 'object'>)
(<class '__main__.C6'>, <class '__main__.C7'>, <type 'object'>)
(<class '__main__.C5'>, <class '__main__.C7'>, <type 'object'>)
(<class '__main__.C4'>, <class '__main__.C7'>, <type 'object'>)
(<class '__main__.C3'>, <class '__main__.C4'>, <class '__main__.C6'>, <class '__main__.C7'>, <type 'object'>)
(<class '__main__.C2'>, <class '__main__.C4'>, <class '__main__.C5'>, <class '__main__.C7'>, <type 'object'>)
(<class '__main__.C1'>, <class '__main__.C2'>, <class '__main__.C3'>, <class '__main__.C4'>, <class '__main__.C5'>, <class '__main__.C6'>, <class '__main__.C7'>, <type 'object'>)
(<class '__main__.C0'>, <class '__main__.C2'>, <class '__main__.C3'>, <class '__main__.C4'>, <class '__main__.C5'>, <class '__main__.C6'>, <class '__main__.C7'>, <type 'object'>)