如何截断列表列表中的每个列表？

Question

I have a list of lists, and in each list I want to truncate all values after the first two.我有一个列表列表，在每个列表中我想截断前两个之后的所有值。

Here is an example with my attempt这是我尝试的一个例子

listss = [ [ 2, 4, 5, 7 ,7], [4, 5,], [2,4 ,4 ,4,4,7,9]]

for item in listss:
    item = item[0:2]

However, this does not alter any of the lists.但是，这不会改变任何列表。

The desired result should look like期望的结果应该是这样的

[ [ 2, 4], [4, 5], [2,4]]

Answer 1

You could do:你可以这样做：

listss = [[2, 4, 5, 7, 7], [4, 5, ], [2, 4, 4, 4, 4, 7, 9]]

for i, item in enumerate(listss):
    listss[i] = item[:2]

print(listss)

Answer 2

You can delete slices in the sub-lists in-place:您可以就地删除子列表中的切片：

for item in listss:
    del item[2:]