将值设置为字典的字典值 [英] Setting a value to a dictionary's dictionary value

问题描述

代码：

>>> mydict = {}
>>> keylist = ['a','b','c']
>>> mydict=dict.fromkeys(keylist,{})
>>> mydict['a']['sample'] = 1
>>> mydict
{'a': {'sample': 1}, 'c': {'sample': 1}, 'b': {'sample': 1}}

我期待着 mydict ['a'] ['sample'] = 1 将这个值设置为一个字典值，并得到这个： {'a'：{'sample'：1}，'c'：{}，'b'：{}} 。

I was expecting mydict['a']['sample'] = 1 would set the value just for a's dictionary value and would get this: {'a': {'sample': 1}, 'c': {}, 'b': {}}.

我在这里缺少什么？

推荐答案

问题是你添加了相同的

mydict = dict((key, {}) for key in keylist)

在上述代码中，您将创建一个新的字典与每个键配对。在您的原始代码中，函数fromkeys接受了参数（您提供的空字典），并添加了这个确切的参数 - 您创建的单个空字典传递给函数 - 到每个键。当那个字典被改变的时候，那个变化就到处都是。

In the above code, you create a new dictionary to pair with each key. In your original code, the function fromkeys took the argument (the empty dictionary you provided) and added that exact argument - that single empty dictionary you created to pass in to the function - to each of the keys. When that one dictionary was changed, then, that change showed up everywhere.

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