python 基础——多重继承

原始的初始化

子类直接调用超类 __init__ 方法初始化，当形成钻石继承的时候，超类会被多次初始化，可能会有意向不到的问题：

BaseClass

/        \

OneClass    TwoClass

\        /

TestClass

from pprint import pprint

class BaseClass(object):
    def __init__(self, a):
        # value = a
        self.value = a

class OneClass(BaseClass):
    def __init__(self, a):
        BaseClass.__init__(self, a)
        # value + 2
        self.value += 2

class TwoClass(BaseClass):
    def __init__(self, a):
        BaseClass.__init__(self, a)
        # value * 2
        self.value *= 2

class TestClass(TwoClass, OneClass):
    def __init__(self, a):
        TwoClass.__init__(self, a)
        OneClass.__init__(self, a)
        # value + 8
        self.value += 8
        print self.value

pprint(TestClass.mro())
test = TestClass(3)    #
# 初始化的过程是：BaseClass ——> TwoClass ——> BaseClass ——> OneClass ——> TestClass
# value 的变化： value = 3 ——>  value = 6 ——> value = 3 ——> value = 5 ——> value = 13

新类初始化

使用 super 可以解决超类重复初始化的问题，保证超类只被初始化一次：

from pprint import pprint

class BaseClass(object):
    def __init__(self, a):
        # value = a
        self.value = a

class OneClass(BaseClass):
    def __init__(self, a):
        super(OneClass, self).__init__(a)
        # value + 2
        self.value += 2

class TwoClass(BaseClass):
    def __init__(self, a):
        super(TwoClass, self).__init__(a)
        # value * 2
        self.value *= 2

class TestClass(TwoClass, OneClass):
    def __init__(self, a):
        super(TestClass, self).__init__(a)
        # value + 8
        self.value += 8
        print self.value

pprint(TestClass.mro())
"""
以以下相反的顺序初始化子类
[<class 'TestClass'>,
 <class 'TwoClass'>,
 <class 'OneClass'>,
 <class 'BaseClass'>,
 <type 'object'>]
"""
test = TestClass(3)    #
# 初始化的过程是：BaseClass ——> OneClass ——> TwoClass ——> TestClass
# value 的变化： value = 3 ——> value = 5 ——> value = 10 ——> value = 18