两层的列表好像有个 bug（要么就是我理解有误…）

[Yvette]

生成一个矩阵（列表内列表，包括 [[]]），复制内容到另一个变量时，虽然 identity 不同，但是内容更改之后还是会联动

>>> n = 2
>>> M = [[0 for _ in range(n)] for _ in range(n)]
>>> m = list(M)
>>> m
[[0, 0], [0, 0]]
>>> M
[[0, 0], [0, 0]]
>>> m[0][0] = 1
>>> m
[[1, 0], [0, 0]]
>>> M
[[1, 0], [0, 0]]
>>>
>>> id(m)-id(M)
1152

将 m = list(M) 换成 m = M[:] 或者 m = M.copy() 也没用

不用列表解析式手打也是一样

>>> M=[[1,2],[]]
>>> m = M.copy()
>>> m
[[1, 2], []]
>>> M
[[1, 2], []]
>>> m[0][0] = 3
>>> m
[[3, 2], []]
>>> M
[[3, 2], []]
>>> 
>>> id(m)-id(M)
960

算法作业查了半个小时才查到这有问题……

[trnhoe]

你需要 m=copy.deepcopy(M)

[xiahei]

```python
In [1]: from copy import deepcopy

In [2]: m = [[1, 2], []]

In [3]: M = deepcopy(m)

In [4]: M
Out[4]: [[1, 2], []]

In [5]: m
Out[5]: [[1, 2], []]

In [6]: m[0][0] = 2

In [7]: m
Out[7]: [[2, 2], []]

In [8]: M
Out[8]: [[1, 2], []]

In [9]: n = 2

In [10]: M = [[0 for _ in range(n)] for _ in range(n)]

In [11]: m = list(M)

In [12]: id(m[0]) == id(M[0])
Out[12]: True
```

[ligyxy]

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

[kindjeff]

是你理解有问题……这样才是对的。

[argsno]

这样是浅复制，只会复制第一层 list 的元素

[Yinz]

外层 list 的 identity 确实不同，但是内层几个对应位置的 list 的 identity 还是一样的 (

[cszhiyue]

http://www.pythontutor.com/live.html#code=import%20copy%0An%20%3D%202%0AM%20%3D%20%5B%5B0%20for%20_%20in%20range%28n%29%5D%20for%20_%20in%20range%28n%29%5D%0Am%20%3D%20list%28M%29%0AM%3D%5B%5B1,2%5D,%5B%5D%5D%0Am%20%3D%20copy.copy%28M%29%0Am%5B0%5D%5B0%5D%20%3D%203%0Am%20%3D%20copy.deepcopy%28M%29&cumulative=true&curInstr=14&heapPrimitives=false&mode=display&origin=opt-live.js&py=2&rawInputLstJSON=%5B%5D&textReferences=false

[Yvette]

@cszhiyue 感谢，这个好，学习了