如何实现__getattribute__而没有无限递归错误? [英] How do I implement __getattribute__ without an infinite recursion error?

问题描述

我想重写对类中一个变量的访问，但通常返回所有其他变量.如何使用__getattribute__完成此操作?

I want to override access to one variable in a class, but return all others normally. How do I accomplish this with __getattribute__?

我尝试了以下操作(这也应该说明我正在尝试执行的操作)，但是出现了递归错误:

I tried the following (which should also illustrate what I'm trying to do) but I get a recursion error:

class D(object):
    def __init__(self):
        self.test=20
        self.test2=21
    def __getattribute__(self,name):
        if name=='test':
            return 0.
        else:
            return self.__dict__[name]

>>> print D().test
0.0
>>> print D().test2
...
RuntimeError: maximum recursion depth exceeded in cmp

推荐答案

您会遇到递归错误，因为您尝试访问__getattribute__内部的self.__dict__属性会再次调用您的__getattribute__.如果您改用object的__getattribute__，它将起作用:

You get a recursion error because your attempt to access the self.__dict__ attribute inside __getattribute__ invokes your __getattribute__ again. If you use object's __getattribute__ instead, it works:

class D(object):
    def __init__(self):
        self.test=20
        self.test2=21
    def __getattribute__(self,name):
        if name=='test':
            return 0.
        else:
            return object.__getattribute__(self, name)

这是有效的，因为object(在此示例中)是基类.通过调用__getattribute__的基本版本，您可以避免以前遇到的递归地狱.

This works because object (in this example) is the base class. By calling the base version of __getattribute__ you avoid the recursive hell you were in before.

Ipython输出，其中包含foo.py中的代码:

Ipython output with code in foo.py:

In [1]: from foo import *

In [2]: d = D()

In [3]: d.test
Out[3]: 0.0

In [4]: d.test2
Out[4]: 21

更新:

标题为当前文档中的对新型类的更多属性访问 ，他们建议完全这样做以避免无限递归.

There's something in the section titled More attribute access for new-style classes in the current documentation, where they recommend doing exactly this to avoid the infinite recursion.

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