转载：https://blog.csdn.net/weixin_42427638/article/details/85261284
看代码时偶然看到的一个写法：
sentences=[y for x in sentences for y in x]
之前没看到过，一头雾水，经过查资料后发现是这样去理解的：
def f(z):
for y in z:
for x in y:
yield x
英文描述：[item for sublist in list for item in sublist]
也就是：子列表中项目的子列表项目
效果展示：
> # flatten a list using a listcomp with two 'for'
> vec = [[1,2,3], [4,5,6], [7,8,9]]
> [num for elem in vec for num in elem]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
作用：将列表展平
当然还有更高级的用法：
[(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
它等价于：
>>> combs = []
>>> for x in [1,2,3]:
... for y in [3,1,4]:
... if x != y:
... combs.append((x, y))
...
>>> combs
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
最后附上python3官方文档连接：https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions