python 中的重定向函数定义

发布于 2024-07-16 08:33:06 字数 954 浏览 7 评论 0原文

这是一个非常人为的示例，因为解释我最终实现该解决方案的上下文并不容易。然而，如果有人能回答为什么会出现这种特殊现象，我将不胜感激。

示例：

class A(dict):  
    def __init__(self):
        self['a'] = 'success'

    def __getitem__(self, name):
        print 'getitem'
        return dict.__getitem__(name)

class B(object):
    def __init__(self):
        self._a = A()
        setattr(self, '__getitem__', self._a.__getitem__) 

b = B()
c = b['a']

此输出：

c = b['a']
TypeError: 'B' object is unsubscriptable

尽管这是一种奇怪的方法（显然子类化会更合乎逻辑），但为什么它找不到我明确设置的方法？

如果我这样做：

dir(b)

我得到这个：

['__class__', '__delattr__', '__dict__', '__doc__', '__getattribute__', '__getitem__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__', '__weakref__', '_a']

其他方法（例如 __iter__）也会出现同样的问题。显式定义这个有效的方法有什么意义？

原文

This is a very contrived example as it's not easy to explain the context in which I have ended up implementing this solution. However, if anyone can answer why this particular peculiarity happens, I'd be grateful.

The example:

class A(dict):  
    def __init__(self):
        self['a'] = 'success'

    def __getitem__(self, name):
        print 'getitem'
        return dict.__getitem__(name)

class B(object):
    def __init__(self):
        self._a = A()
        setattr(self, '__getitem__', self._a.__getitem__) 

b = B()
c = b['a']

This outputs:

c = b['a']
TypeError: 'B' object is unsubscriptable

Even though it's a bizarre way of doing this (clearly subclassing would be more logical), why doesn't it find the method I have explicitly set?

If I do this:

dir(b)

I get this:

['__class__', '__delattr__', '__dict__', '__doc__', '__getattribute__', '__getitem__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__', '__weakref__', '_a']

The same issue occurs with other methods such as __iter__. What is it about explicitly defining this method that works?

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￡噩梦荏苒 2024-07-23 08:33:06

当你使用方括号[]时，Python会在类中查找。您必须在类中设置该方法。

这是您的代码：

class A(dict):  
    def __init__(self):
        self['a'] = 'success'

    def __getitem__(self, name):
        print 'getitem!'
        return dict.__getitem__(self, name)

class B(object):
    def __init__(self):
        self._a = A()
        B.__getitem__ = self._a.__getitem__

b = B()
c = b['a']

When you use brackets [] python looks in the class. You must set the method in the class.

Here's your code adapted:

class A(dict):  
    def __init__(self):
        self['a'] = 'success'

    def __getitem__(self, name):
        print 'getitem!'
        return dict.__getitem__(self, name)

class B(object):
    def __init__(self):
        self._a = A()
        B.__getitem__ = self._a.__getitem__

b = B()
c = b['a']

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