appending to a nested list in python [duplicate]

问题

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Python behaves unexpected when i append to a list, which is in another list. Here's an example:

>>> _list = [[]] * 7
>>> _list
[[], [], [], [], [], [], []]
>>> _list[0].append("value")

What i expect:

>>> _list
[['value'], [], [], [], [], [], []]

What i get:

>>> _list
[['value'], ['value'], ['value'], ['value'], ['value'], ['value'], ['value']]

Why is this? how can i go around it?

回答1:

Your problem is that your list does not contain seven independent lists, but rather the same list seven times.

To create a list of list, better use a list comprehension:

_list = [[] for _ in xrange(7)]

which will result in a list containing seven different lists.

来源：https://stackoverflow.com/questions/13763157/appending-to-a-nested-list-in-python