在一些时候，我们经常需要对字典进行嵌套赋值，但是每次取出来还要判断是否存在，所以我们希望有一种方法能够方便的对字典进行初始化。。以下是字典嵌套初始化的方法。
def pack_data():
dic = {}
list1=[1,2,3]
list2=[4,5,6]
list3=[7,8,9]
for layer1 in list1:
dic[layer1] = {}.fromkeys(list2)
for layer2 in list2:
dic[layer1][layer2] = {}.fromkeys(list3)
for layer3 in list3:
dic[layer1][layer2][layer3]=5
return dic
'''
得到的结果是:
{1: {4: {8: 5, 9: 5, 7: 5},
5: {8: 5, 9: 5, 7: 5},
2: {4: {8: 5, 9: 5, 7: 5},
5: {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}}}
'''
