理解Python中super

文章作者：Tyan
博客：noahsnail.com | CSDN | 简书

1. 单继承

在单继承时，super().__init__()与Base.__init__()是一样的。super()避免了基类的显式调用。代码如下：

super().init()

class Base(object):
    def __init__(self):
        print 'Create Base'

class A(Base):
    def __init__(self):
        Base.__init__(self)
        print 'Create A'

A()

# 测试结果
Create Base
Create A

Base.init()

class Base(object):
    def __init__(self):
        print 'Create Base'

class A(Base):
    def __init__(self):
        super(A, self).__init__()
        # super().__init()  python3
        print 'Create A'

A()

# 测试结果
Create Base
Create A

2. 多继承

super与父类没有实质性的关联。在单继承时，super获取的类刚好是父类，但多继承时，super获取的是继承顺序中的下一个类。以下面的继承方式为例：

  Base
  /  \
 /    \
A      B
 \    /
  \  /
   C

使用super，代码如下：

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

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

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

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

C()

# 测试结果
enter C
enter A
enter B
enter Base
leave Base
leave B
leave A
leave C

如果不使用super，代码如下：

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

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

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

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

C()

# 测试结果
enter C
enter A
enter Base
leave Base
enter Base
leave Base
leave A
enter A
enter Base
leave Base
enter Base
leave Base
leave A
enter B
enter Base
leave Base
enter Base
leave Base
leave B
enter B
enter Base
leave Base
enter Base
leave Base
leave B
leave C

从上面可以看到如果不使用super，会导致基类被多次调用，开销非常大。

对于定义的类，在Python中会创建一个MRO(Method Resolution Order)列表，它代表了类继承的顺序。查看MRO列表：

使用super

class Base(object):
    def __init__(self):
        print "Create Base"

class A(Base):
    def __init__(self):
        super(A, self).__init__()
        print "Create A"

class B(Base):
    def __init__(self):
        super(B, self).__init__()
        print "Create B"

class C(A, B):
    def __init__(self):
        super(C, self).__init__()
        print "Create C"

print C.mro()

# 测试结果

[<class '__main__.C'>, <class '__main__.A'>, <class '__main__.B'>, <class '__main__.Base'>, <type 'object'>]

不使用super

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

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

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

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

print C.mro()

# 测试结果
[<class '__main__.C'>, <class '__main__.A'>, <class '__main__.B'>, <class '__main__.Base'>, <type 'object'>]

从测试结果来看，两种方式的MRO列表是一样的。MRO的查找顺序是按广度优先来的(基类继承object，Python 2.3之后)。

1. 单继承

2. 多继承

参考资料