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Python中super()解析

13 March 2015

super

super(type[, object-or-type])

返回一个代理类来执行父类或者兄弟类的方法. 如果参数object-or-type省略，返回的对象不会绑定，如果object-or-type是一个对象，那么函数isinstance(obj, type) 必须为TRUE，如果object-or-type，那么issubclass(type2, type)必须为TRUE

class A(object):   # 只能是new-style class
    def __init__(self):
        print "enter A"
        print "leave A"

class B(A):
    def __init__(self):
        print "enter B"
        print super(B, self)
        super(B, self).__init__()
        print "leave B"

b = B()

结果：

enter B
<super: <class 'B'>, <B object>> # 看到了super执行后的类型
enter A
leave A
leave B

为什么会返回兄弟类？看下面的例子：

class A(object):
    def __init__(self):
        print "enter A"
        print "leave A"

class B(object):
    def __init__(self):
        print "enter B"
        print "leave B"

class C(A):
    def __init__(self):
        print "enter C"
        super(C, self).__init__()
        print "leave C"

class D(A):
    def __init__(self):
       print "enter D"
       super(D, self).__init__()
       print "leave D"

class E(B, C):
    def __init__(self):
        print "enter E"
        B.__init__(self)
        C.__init__(self)
        print "leave E"

class F(E, D):
    def __init__(self):
        print "enter F"
        E.__init__(self)
        D.__init__(self)
        print "leave F"

F()

输出：

 enter F
 enter E
 enter B
 leave B
 enter C
 enter D
 enter A
 leave A
 leave D
 leave C
 leave E
 enter D
 enter A
 leave A
 leave D
 leave F

类A和类D的初始化函数被重复调用了2次，C的调用为什么会调用D的__init__？

        object
        /  \
       /    A
      /    /  \    
      B   C    D
      \  /    /
       E     /
        \   /
          F

这里涉及一个叫做Method resolution order(mro)的概念，super是以__mro__的顺序来创建的，这个图的mro顺序是：

 F E B C D A object

调用super(C, self).__init__()的时候会去找C前面的，就是D了，因此调用了D的__init__

如果让F E B C D A object类构造只调用一遍，从F-A依次调用super即可

A,B，E，F类中调用

class A(object):
    def __init__(self):
        print "enter A"
        super(A, self).__init__()
        print "leave A"

class B(object):
    def __init__(self):
        print "enter B"
        super(B, self).__init__()
        print "leave B"
#....
class E(B, C):
    def __init__(self):
        print "enter E"
        super(E, self).__init__()
        print "leave E"

class F(E, D):
    def __init__(self):
        print "enter F"
        super(F, self).__init__()
        print "leave F"

参考资料