为什么我不能只更改Python嵌套列表中的单个元素 [英] why can't I change only a single element in a nested list in Python

问题描述

我刚刚遇到了一些Python的真正奇怪的东西:

I just met something really strange of Python:

>>> out=[[0]*3]*3
>>> out
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> out[0][1]
0
>>> out[0][1]=9
>>> out
[[0, 9, 0], [0, 9, 0], [0, 9, 0]]

好吧，很明显，我想要的是:

well, obviously, what I want is :

[[0, 9, 0], [0, 0, 0], [0, 0, 0]]

不是很奇怪吗?我对Python不太熟悉，但是Python的直观行为总是给我留下深刻的印象.但是，这是怎么回事呢?
...以及如何获得所需的东西?

isn't strange? I'm not very familiar with Python, but Python always impresses me with its intuitive behavior. But how it comes up with this?
... and how can I get what I need?

谢谢！

瓦特

推荐答案

确实是一个奇怪的行为，但这仅是因为*运算符会生成浅表副本(在您的情况下)是[0, 0, 0]列表的浅表副本.您可以使用id()函数来确保这些内部列表实际上是相同的:

A strange behaviour indeed, but that's only because * operator makes shallow copies, in your case - shallow copies of [0, 0, 0] list. You can use the id() function to make sure that these internal lists are actually the same:

out=[[0]*3]*3
id(out[0])
>>> 140503648365240
id(out[1])
>>> 140503648365240
id(out[2])
>>> 140503648365240

理解可以用来创建不同列表，如下所示:

Comprehensions can be used to create different lists as follows:

out = [ [0]*3 for _ in range(3) ]

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