Python双层嵌套循环list生成式

老是忘记前后顺序，简单写个笔记

import numpy as np

a = np.arange(1, 13).reshape(3, 4)
a

array([[ 1,  2,  3,  4],
       [ 5,  6,  7,  8],
       [ 9, 10, 11, 12]])

for i in a:
    for j in i:
        print(j, end=" ")

1 2 3 4 5 6 7 8 9 10 11 12

[j for i in  a for j in i] # 外层在前，内层在后

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]