Python dict.fromkeys返回相同的id元素

问题描述

当我在python上执行此操作时，我会一次更新所有密钥.

When i do this on python i update all keys in one time.

>>> base = {}
>>> keys = ['a', 'b', 'c']
>>> base.update(dict.fromkeys(keys, {}))
>>> base.get('a')['d'] = {}
>>> base
{'a': {'d': {}}, 'c': {'d': {}}, 'b': {'d': {}}}
>>> map(id, base.values())
[140536040273352, 140536040273352, 140536040273352]

如果我使用[]运算符而不是.get，则不会发生这种情况:

If instead of .get i use [] operator this not happen:

>>> base['a']['d'] = {}
>>> base
{'a': {'d': {}}, 'c': {}, 'b': {}}

为什么?

推荐答案

当您将新键的值初始化为{}时，将创建一个新的词典，并且对该词典的引用正在成为这些值.只有一本字典，因此，如果您更改一本字典，则将更改全部".

When you initialize the value for the new keys as {} a new dictionary is created and a reference to this dictionary is becoming the values. There is only one dictionary and so if you change one, you will change "all".

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