在python中创建默认列表 [英] creating a defaultlist in python

问题描述

我正在尝试为非常有用的 collections.defaultdict 创建一个等效列表.以下设计效果很好:

I'm trying to create a list equivalent for the very useful collections.defaultdict. The following design works nicely:

class defaultlist(list):
    def __init__(self, fx):
        self._fx = fx
    def __setitem__(self, index, value):
        while len(self) <= index:
            self.append(self._fx())
        list.__setitem__(self, index, value)

使用方法如下:

>>> dl = defaultlist(lambda:'A')
>>> dl[2]='B'
>>> dl[4]='C'
>>> dl
['A', 'A', 'B', 'A', 'C']

我应该在默认列表中添加些什么，以支持以下行为?

What should I add to the defaultlist so as to support the following behavior?

>>> dl = defaultlist(dict)
>>> dl[2]['a'] = 1
>>> dl
[{}, {}, {'a':1}]

推荐答案

在给出的示例中，您首先尝试检索列表中不存在的值，就像执行dl[2]['a']一样，Python首先检索第三个(索引2)元素，然后继续在该对象上获取名为'a'的元素-因此，您还必须实现对__getitem__方法的自动扩展行为，如下所示:

On the example you give, you first try to retrieve a non-existing value on the list, as you do dl[2]['a'], Python first retrieve the third (index 2) element on the list, then proceed to get the element named 'a' on that object - therefore you have to implement your automatic extending behavior to the __getitem__ method as well, like this:

class defaultlist(list):
    def __init__(self, fx):
        self._fx = fx
    def _fill(self, index):
        while len(self) <= index:
            self.append(self._fx())
    def __setitem__(self, index, value):
        self._fill(index)
        list.__setitem__(self, index, value)
    def __getitem__(self, index):
        self._fill(index)
        return list.__getitem__(self, index)

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