python 字典嵌套赋值

>>> def foo():

...     d = {}

...     list_1 = [1,2,3]

...     list_2 = [4,5,6]

...     list_3 = [7,8,9]

...     for key1 in list_1:

...         d[key1] = {}.fromkeys(list_2)

...         for key2 in list_2:

...             d[key1][key2] = {}.fromkeys(list_3)

...             for key3 in list_3:

...                 d[key1][key2][key3] = 5

...     return d

... 
>>> 

其中fromkeys的作用：

>>> {}.fromkeys(list_2)

{4: None, 5: None, 6: None}

>>> foo()

{1: {4: {8: 5, 9: 5, 7: 5}, 5: {8: 5, 9: 5, 7: 5}, 6: {8: 5, 9: 5, 7: 5}}, 2: {4: {8: 5, 9: 5, 7: 5}, 5: {8: 5, 9: 5, 7: 5}, 6: {8: 5, 9: 5, 7: 5}}, 3: {4: {8: 5, 9: 5, 7: 5}, 5: {8: 5, 9: 5, 
7: 5}, 6: {8: 5, 9: 5, 7: 5}}}

>>> list_1 = [1,2,3]

>>> list_2 = [4,5,6]
>>> list_3 = [7,8,9]

下边是简写方式：

>>> {k1:{k2:{k3:5 for k3 in list_3} for k2 in list_2} for k1 in list_1}

{1: {4: {8: 5, 9: 5, 7: 5}, 5: {8: 5, 9: 5, 7: 5}, 6: {8: 5, 9: 5, 7: 5}}, 2: {4: {8: 5, 9: 5, 7: 5}, 5: {8: 5, 9: 5, 7: 5}, 6: {8: 5, 9: 5, 7: 5}}, 3: {4: {8: 5, 9: 5, 7: 5}, 5: {8: 5, 9: 5, 
7: 5}, 6: {8: 5, 9: 5, 7: 5}}}

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