python - 为什么没有调用所有基类构造函数？

Question

在 Python 2.7.10 中

class OneMixin(object):
    def __init__(self):
        # super(OneMixin, self).__init__()
        print "one mixin"

class TwoMixin(object):
    def __init__(self):
        # super(TwoMixin, self).__init__()
        print "two mixin"

class Account(OneMixin, TwoMixin):
    def __init__(self):
        super(Account, self).__init__()
        print "account"

Account.mro() 是:[<class 'Account'>, <class 'OneMixin'>, <class 'TwoMixin'>, <type 'object'>]

尽管 MRO 中列出了每个类，但并未打印“两个 mixin”。

如果我取消注释 OneMixin 和 TwoMixin 中的 super 调用，MRO 完全相同，但会打印“两个 mixin”。

为什么有区别？我希望 MRO 中的所有内容都会被调用。

Answer 1

这是因为 super 用于将调用委托(delegate)给某个类型的父类或同级类。 Python documentation第二个用例的描述如下:

The second use case is to support cooperative multiple inheritance in a dynamic execution environment. This use case is unique to Python and is not found in statically compiled languages or languages that only support single inheritance. This makes it possible to implement “diamond diagrams” where multiple base classes implement the same method. Good design dictates that this method have the same calling signature in every case (because the order of calls is determined at runtime, because that order adapts to changes in the class hierarchy, and because that order can include sibling classes that are unknown prior to runtime).

如果您从 OneMixin 中删除 super 调用，则不会将该调用委托(delegate)给 MRO 中的下一个类型。