Python3 a for b in c for a in b的用法 | 列表的嵌套

用于列表的嵌套中
#先遍历c，再遍历b，把其中的元素a存到列表中

c = [[7,8,9],[1,2,3],[4,5,6]]
l = [a for b in c for a in b]
print(l)

# [7, 8, 9, 1, 2, 3, 4, 5, 6]

等价于：

c = [[7,8,9],[1,2,3],[4,5,6]]
k = []
for b in c:
    for a in b:
        k.append(a)
print(k)