追加到嵌套列表列出的清单内

Question

考虑这个例子：

>>> result = [[]] * 8 
>>> result 
[[], [], [], [], [], [], [], []] 
>>> result[0] 
[] 
>>> result[0].append("foo") 
>>> result # wtf? expected result: [['foo'], [], [], [], [], [], [], []] 
[['foo'], ['foo'], ['foo'], ['foo'], ['foo'], ['foo'], ['foo'], ['foo']] 

我非常这一困惑。也许我不明白如何使用append。我将如何附加到列表中的i嵌套列表中？

Answer 1

这是因为，这样做：

result = [[]] * 8 

你让8个复制相同名单。您的代码应该是：

>>> result = [[] for _ in xrange(8)] 
>>> result 
[[], [], [], [], [], [], [], []] 
>>> result[0] 
[] 
>>> result[0].append("foo") 
>>> result 
[['foo'], [], [], [], [], [], [], []] 
>>> 

作为一个证明，看看这个：

>>> lst = [[]] * 2 
>>> lst 
[[], []] 
>>> id(lst[0]) 
28406048 
>>> id(lst[1]) 
28406048 
>>> 

注意，列出的ID是因为在这些地方一样：

>>> lst = [[] for _ in xrange(2)] 
>>> lst 
[[], []] 
>>> id(lst[0]) 
28408408 
>>> id(lst[1]) 
28418096 
>>> 

它们是不同的。