如何自定义Python的方法解析顺序(mro)？

Question

我想自定义调用我继承的方法。

这是一个示例代码：

class First(object):
    def get(self):
        print('getting from first')

    def set(self):
        print('setting to first')

class Second(object):
    def get(self):
        print('getting from second')

    def set(self):
        print('setting to second')

class Third(First, Second):
    def get(self):
        super(Third, self).get()

    def set(self):
        super(Third, self).set()

现在我想要的行为是：

third = Third()
third.get() # -> should print 'getting from first'
third.set() # -> should print 'setting to second'

现在 mro 显示：

Third.__mro__ ->  (__main__.Third, __main__.First, __main__.Second, object)

我们可以看到 main.First 中的方法总是先被调用。虽然我想要的是 main.Second 在执行 set() 方法期间首先被调用。

这是我解决这个问题的尝试，尝试修改 Third 的 MRO class:

思路是把两个class的两个位置互换一下，看看能不能行得通。首先，一个 swap() 辅助函数。

def swap(index1, index2, mro_tuple):
    l = list(mro_tuple)
    temp = l[index1]
    l[index1] = l[index2]
    l[index2] = temp
    return tuple(l)

然后在 set() 方法的实现过程中，我尝试修改底层 class 的 mro .

class Third(First, Second):
    def get(self):
        super(Third, self).get()
    def set(self):
        self.__class__.__mro__ = swap(1, 2, self.__class__.__mro__) # swap here..
        super(Third, self).set() # then call method**
In [43]: third = Third() 

In [44]: third.get()                                                            
getting from first

In [45]: third.set()                                                            
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-53-c82ac1a0d5bc> in <module>
----> 1 third.set()

<ipython-input-50-00c9baff0d57> in set(self)
      4 
      5     def set(self):
----> 6         self.__class__.__mro__ = swap(1, 2, self.__class__.__mro__) # swap here..
      7         super(Third, self).set() # then call method
      8 

AttributeError: readonly attribute

说明无法重置__mro__属性

有没有一种方便的方式来实现这种行为？

Answer 1

您最好的选择可能是 Third 明确使用 Second 实现：

class Third(First, Second):
    set = Second.set

虽然您问这个问题是一个警告信号，表明您可能选择了错误的 class 层次结构。

如何自定义Python的方法解析顺序(mro)？

How to customize the method resolution order (mro) of Python?

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multiple-inheritance

method-resolution-order