在Python中附加到2D列表 [英] Appending to 2D lists in Python

问题描述

我遇到了我认为在Python中很奇怪的行为，如果可能的话，我希望有人来解释它.

I've encountered what I think is a strange behavior in Python, and I'd like somebody to explain it if possible.

我创建了一个空的2D列表

I've created an empty 2D list

listy = [[]]*3

print listy

[[], [], []]

以下功能符合我的预期:

The following works as I'd expect:

listy[1] = [1,2]产生[[], [1,2], []]

listy[1].append(3)产生[[], [1,2,3], []]

但是，当我追加到一个空列表之一时，python追加到所有子列表中，如下所示:

However, when I append to one of the empty lists, python appends to ALL of the sublists, as follows:

listy[2].append(1)产生[[1], [1,2,3], [1]].

任何人都可以向我解释为什么会发生这种行为吗?

Can anyone explain to me why this behavior occurs?

推荐答案

您尚未创建三个不同空列表.您已经创建了一个一个空列表，然后创建了一个新列表，其中包含对该 same 空列表的三个引用.要解决此问题，请改用以下代码:

You haven't created three different empty lists. You've created one empty list, and then created a new list with three references to that same empty list. To fix the problem use this code instead:

listy = [[] for i in range(3)]

现在运行示例代码即可得到您可能期望的结果:

Running your example code now gives the result you probably expected:

>>> listy = [[] for i in range(3)]
>>> listy[1] = [1,2]
>>> listy
[[], [1, 2], []]
>>> listy[1].append(3)
>>> listy
[[], [1, 2, 3], []]
>>> listy[2].append(1)
>>> listy
[[], [1, 2, 3], [1]]

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