appending to a nested list in python

Question

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?

Answer 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.