>>> 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]

下邊是簡寫方式：

>>> {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}}}

來自python群