Python 多重继承顺序

本文借鉴自 http://www.cnblogs.com/panyinghua/p/3283726.html

当类有多个超类，而且超类中有相同的方法时，继承顺序就很重要，如下：

        class A
        (def save(self):...)    # A 中有self方法
        /       \
       /          \
class B         class C
                (def save(self):...)    # B 和 C 中都继承 A，只有C中重写save方法
      \         /
       \          /
        \       /
         class D        # 继承 B 和 C

旧式类

在旧式类中，会根据深度优先(depth-first)的顺序来继承，由左至右依次查找save()方法，D -> B -> A ->C,当查到方法时，立刻放回，不在继续查找。

# 例：
class A:
    def __init__(self):
        print 'it is A'
    def save(self):
        print 'save method from A'

class B(A):
    def __init__(self):
        print 'it is B'

class C(A):
    def __init__(self):
        print 'it is C'
    def save(self):
        print 'save method from C'

class D(B, C):
    def __init__(self):
        print 'it is D'

d = D()
d.save()
# 结果 ==》
it is D
save method from A  # 根据上边的顺序，查到A中的save方法，即返回。

新式类

新式类中会根据方法解析顺序（MRO）来进行搜索，广度优先，当查到对应方法属性时，立即返回，不再继续查找。

__mro__属性，标记了python继承层次中父类的查找顺序，Python的多重继承机制就是按照__mro__的顺序进行查找，一旦找到对应属性，则查找马上返回。

# 例 1
class A(object):
    def __init__(self):
        print 'it is A'
    def save(self):
        print 'save method from A'

class B(A):
    def __init__(self):
        print 'it is B'

class C(A):
    def __init__(self):
        print 'it is C'
    def save(self):
        print 'save method from C'

class D(B, C):
    def __init__(self):
        print 'it is D'

print D.__mro__
d = D()
d.save()
# 结果==》
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <type 'object'>)
it is D
save method from C

# 例 2
class A(object):
    def save(self):
        print ('save A')

class B(object):
    def save(self):
        print ('save B')

class C(object):
    def save(self):
        print ('save C')

class D(B,C,A):
    pass

print D.__mro__
cla = D()
cla.save()
# 结果
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <type 'object'>)
save B

# 例 3
class B(object):
    def foo(self):
        print ('foo B')

    def bar(self):
        print 'bar B'

class A(object):
    def foo(self):
        print ('foo A')

class C1(A, B):
    pass

class C2(A, B):
    def bar(self):
        print 'bar C2'

class D(C1, C2):
    pass

print D.__mro__
d = D()
d.foo()
d.bar()

# 结果
(<class '__main__.D'>, <class '__main__.C1'>, <class '__main__.C2'>, <class '__main__.A'>, <class '__main__.B'>, <type 'object'>)
foo A
bar C2