清单清单的Python切片 [英] Python slicing of list of list

问题描述

说我有一个列表清单

    >>> s = [ [1,2], [3,4], [5,6] ]

我可以访问第二个列表中的项目:

I can access the items of the second list:

    >>> s[1][0]
    3
    >>> s[1][1]
    4

整个第二个列表为:

    >>> s[1][:]
    [3, 4]

但是下面的原因为什么也给我第二个列表?

But why does the following give me the second list as well?

    >>> s[:][1]
    [3, 4]

我认为这会给我三个列表中每一个的第二项.

I thought it would give me the second item from each of the three lists.

一个人可以使用列表理解来实现这一点(如问题13380993 )，但我很好奇如何正确理解s[:][1].

One can use list comprehension to achieve this (as in question 13380993), but I'm curious how to properly understand s[:][1].

推荐答案

s[:]返回列表的副本.下一个[...]适用于先前返回的表达式，而[1]仍然是该列表的第二个元素.

s[:] returns a copy of a list. The next [...] applies to whatever the previous expression returned, and [1] is still the second element of that list.

如果您想拥有第二个项目，请使用列表理解:

If you wanted to have every second item, use a list comprehension:

[n[1] for n in s]

演示:

>>> s = [ [1,2], [3,4], [5,6] ]
>>> [n[1] for n in s]
[2, 4, 6]

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