Python 2 __missing__方法 [英] Python 2 __missing__ method

问题描述

我写了一个非常简单的程序来对字典进行子类化.我想尝试python中的 __ missing __ 方法.经过一番研究，我发现在Python 2中，它可以在 defaultdict 中使用.(虽然在python 3中，我们使用collections.UserDict ..)如果找不到密钥，则 __ getitem __ 负责调用 __ missing __ 方法.

I wrote a very simple program to subclass a dictionary. I wanted to try the __missing__ method in python. After some research i found out that in Python 2 it's available in defaultdict. ( In python 3 we use collections.UserDict though..) The __getitem__ is the on responsible for calling the __missing__ method if the key isn't found.

当我在以下程序中实现 __ getitem __ 时，我遇到了关键错误，但是当我在没有它的情况下实现时，我得到了所需的值.

When i implement __getitem__ in the following program i get a key error, but when i implement without it, i get the desired value.

import collections
class DictSubclass(collections.defaultdict):

    def __init__(self,dic):
        if dic is None:
            self.data = None
        else:
            self.data = dic

    def __setitem__(self,key,value):
        self.data[key] = value

    ########################
    def __getitem__(self,key):
        return self.data[key]
    ########################

    def __missing__(self,key):
        self.data[key] = None

dic = {'a':4,'b':10}
d1 = DictSubclass(dic)
d2 = DictSubclass(None)    
print  d1[2]

我认为我需要实现 __ getitem __ ，因为它负责调用 __ missing __ .我了解defaultdict的类定义具有 __ getitem __ 方法.但是即使如此，说我想写自己的 __ getitem __ ，我该怎么做?

I thought i needed to implement __getitem__ since it's responsible for calling __missing__. I understand that the class definition of defaultdict has a __getitem__ method. But even so, say i wanted to write my own __getitem__, how would i do it?

推荐答案

dict 类型将总是尝试调用 __ missing __ . defaultdict 所做的只是提供一个实现.如果您要提供自己的 __ missing __ 方法，则不必完全继承 defaultdict 的子类.

The dict type will always try to call __missing__. All that defaultdict does is provide an implementation; if you are providing your own __missing__ method you don't have to subclass defaultdict at all.

请参见 dict 文档:

d [key]
用键 key 返回 d 的项目.如果 key 不在地图中，则引发 KeyError .

d[key]
Return the item of d with key key. Raises a KeyError if key is not in the map.

如果dict的子类定义了方法 __ missing __()并且 key 不存在，则 d [key] 操作将调用该方法以键 key 作为参数.然后， d [key] 操作将返回或引发 __ missing __(key)调用所返回或引发的内容.没有其他操作或方法调用 __ missing __().

If a subclass of dict defines a method __missing__() and key is not present, the d[key] operation calls that method with the key key as argument. The d[key] operation then returns or raises whatever is returned or raised by the __missing__(key) call. No other operations or methods invoke __missing__().

但是，您需要保留默认的 __ getitem __ 方法，或者至少调用它.如果您使用自己的版本覆盖 dict.__getitem__ 并且不调用基本实现，则永远不会调用 __missing__.

However, you need to leave the default __getitem__ method in place, or at least call it. If you override dict.__getitem__ with your own version and not call the base implementation, __missing__ is never called.

您可以从自己的实现中调用 __ missing __ :

You could call __missing__ from your own implementation:

def __getitem__(self, key):
    if key not in self.data:
        return self.__missing__(key)
    return self.data[key]

或者您可以调用原始实现:

or you could call the original implementation:

def __getitem__(self, key):
    if key not in self.data:
        return super(DictSubclass , self).__getitem__(key)
    return self.data[key]

在Python 2中，您可以仅继承 UserDict.UserDict :

In Python 2, you can just subclass UserDict.UserDict:

from UserDict import UserDict

class DictSubclass(UserDict):
    def __missing__(self, key):
        self.data[key] = None

这篇关于Python 2 __missing__方法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！