python-2.7 尝试修改单个值时，2D 列表有奇怪的行为
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2D list has weird behavor when trying to modify a single value
提问by Brian
Possible Duplicate:
Unexpected feature in a Python list of lists
可能的重复：
Python 列表中的意外功能
So I am relatively new to Python and I am having trouble working with 2D Lists.
所以我对 Python 比较陌生，并且在使用 2D 列表时遇到了麻烦。
Here's my code:
这是我的代码：
data = [[None]*5]*5
data[0][0] = 'Cell A1'
print data
and here is the output (formatted for readability):
这是输出（格式化为可读性）：
[['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]]
Why does every row get assigned the value?
为什么每一行都被赋值？
回答by Mark Byers
This makes a list with five references to the samelist:
这将生成一个列表，其中包含对同一列表的五个引用：
data = [[None]*5]*5
Use something like this instead which creates five separate lists:
使用类似这样的东西来创建五个单独的列表：
>>> data = [[None]*5 for _ in range(5)]
Now it does what you expect:
现在它可以满足您的期望：
>>> 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]]
回答by user151019
As the python library reference for sequence types, which includes lists, says
Note also that the copies are shallow; nested structures are not copied. This often haunts new Python programmers; consider:
另请注意，副本很浅；不复制嵌套结构。这常常困扰着新的 Python 程序员；考虑：
>>> 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.
发生的事情是 [[]] 是一个包含空列表的单元素列表，因此 [[]] * 3 的所有三个元素都是（指向）这个单个空列表。修改列表的任何元素都会修改这个单个列表。
You can create a list of different lists this way:
您可以通过以下方式创建不同列表的列表：
>>> lists = [[] for i in range(3)]
>>> lists[0].append(3)
>>> lists[1].append(5)
>>> lists[2].append(7)
>>> lists
[[3], [5], [7]]
回答by lfagundes
In python every variable is an object, and so a reference. You first created an array of 5 Nones, and then you build an array with 5 times the same object.
在python中，每个变量都是一个对象，因此是一个引用。您首先创建了一个包含 5 个 None 的数组，然后构建了一个包含 5 个相同对象的数组。
