python中的list的*运算使用过程中遇到的问题

目的:

想生成一个[[],[],[]] 这样的列表,

所以就 [[]]*3 这样做了,但是这样做会有问题,这样list中的三个list其实是同一个list。

例如:a=[[]]*3,然后a[0].append(1),

然后a就变成这样了:[[1],[1],[1]]

验证一下,发现表达式 a[0] is a[1] 的值为True。

如何解决呢,可以用列表生成器:a=[[] for i in range(3)]

这应该像是值类型和引用类型的区别,但是翻看python的文档时没发现有类似的说法,不过在翻看文档时发现里面提到了这个情形:

https://docs.python.org/3.6/library/stdtypes.html

内容摘录如下:

Note that items in the sequence s are not copied; they are referenced multiple times. This often haunts new Python programmers; consider:

>>>
>>> lists = [[]] * 3
>>> lists
[[], [], []]
>>> lists[0].append(3)
>>> lists
[[3], [3], [3]]

What has happened is that [[]] is a one-element list containing an empty list, so all three elements of [[]] * 3 are references to this single empty list. Modifying any of the elements of lists modifies this single list. You can create a list of different lists this way:

>>>
>>> lists = [[] for i in range(3)]
>>> lists[0].append(3)
>>> lists[1].append(5)
>>> lists[2].append(7)
>>> lists
[[3], [5], [7]]
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