2D list has weird behavor when trying to modify a single value
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Possible Duplicate:
Unexpected feature in a Python list of lists
所以我对python比较陌生，在使用二维列表时遇到了困难。
以下是我的代码：
1 2 3 | data = [[None]*5]*5 data[0][0] = 'Cell A1' print data |
下面是输出(格式化为可读性)：
1 2 3 4 5 | [['Cell A1', None, None, None, None], ['Cell A1', None, None, None, None], ['Cell A1', None, None, None, None], ['Cell A1', None, None, None, None], ['Cell A1', None, None, None, None]] |
为什么每一行都被赋值？
这将生成一个列表，其中包含对同一列表的五个引用：
1 | data = [[None]*5]*5 |
使用类似这样的方法来创建五个单独的列表：
1 | >>> data = [[None]*5 for _ in range(5)] |
现在它实现了您的期望：
1 2 3 4 5 6 7 | >>> data[0][0] = 'Cell A1' >>> print data [['Cell A1', None, None, None, None], [None, None, None, None, None], [None, None, None, None, None], [None, None, None, None, None], [None, None, None, None, None]] |
作为序列类型(包括列表)的python库引用，
Note also that the copies are shallow; nested structures are not copied. This often haunts new Python programmers; consider:
1 2 3 4 5 6 | >>> 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 (pointers to) this single empty list. Modifying any of the elements of lists modifies this single list.
您可以通过以下方式创建不同列表的列表：
1 2 3 4 5 6 | >>> lists = [[] for i in range(3)] >>> lists[0].append(3) >>> lists[1].append(5) >>> lists[2].append(7) >>> lists [[3], [5], [7]] |
在Python中，每个变量都是一个对象，因此是一个引用。首先创建一个由5个nones组成的数组，然后用同一对象的5倍构建一个数组。