python 字典值更新

Question

我有以下变量：

list_m = ["a","b","c"]
list_s = ['x','y','z']
dict_m = dict.fromkeys(list_m[:])
dict_s = dict.fromkeys(list_s[:],copy.deepcopy(dict_m)) # empty dict of dicts

所以我有

In[22]: dict_s
Out[22]: 
{'x': {'a': None, 'b': None, 'c': None},
 'y': {'a': None, 'b': None, 'c': None},
 'z': {'a': None, 'b': None, 'c': None}}

像这样更新 dict_s 的值

 dict_s['x']['a']= np.arange(10)

我明白了

In[27]: dict_s
Out[27]: 
{'x': {'a': array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]), 'b': None, 'c': None},
 'y': {'a': array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]), 'b': None, 'c': None},
 'z': {'a': array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]), 'b': None, 'c': None}}

而不是我 wanted/expected:

In[27]: dict_s
Out[27]: 
{'x': {'a': array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]), 'b': None, 'c': None},
 'y': {'a': None, 'b': None, 'c': None},
 'z': {'a': None, 'b': None, 'c': None}}

我不太明白这是 deep/shallow 复制问题还是其他问题。

Answer 1

fromkeys 对每个键使用相同的默认值。如果你想要单独的值，你可以使用字典理解并为每个值生成新的字典 fromkeys:

>>> list_m = ["a","b","c"]
>>> list_s = ['x','y','z']
>>> dict_s = {x: dict.fromkeys(list_m) for x in list_s}
>>> dict_s
{'y': {'a': None, 'c': None, 'b': None}, 'x': {'a': None, 'c': None, 'b': None}, 'z': {'a': None, 'c': None, 'b': None}}
>>> dict_s['y']['a'] = 100
>>> dict_s
{'y': {'a': 100, 'c': None, 'b': None}, 'x': {'a': None, 'c': None, 'b': None}, 'z': {'a': None, 'c': None, 'b': None}}

python 字典值更新

python dictionary value update

python

dictionary

pass-by-reference

deep-copy