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]]
参考文献: zhang