Appending to 2D lists in Python [duplicate]

Question

I've encountered what I think is a strange behavior in Python, and I'd like somebody to explain it if possible.

I've created an empty 2D list

listy = [[]]*3  print listy  [[], [], []] 

The following works as I'd expect:

listy[1] = [1,2] yields [[], [1,2], []]

listy[1].append(3) yields [[], [1,2,3], []]

However, when I append to one of the empty lists, python appends to ALL of the sublists, as follows:

listy[2].append(1) yields [[1], [1,2,3], [1]].

Can anyone explain to me why this behavior occurs?

Answer 1

You haven't created three different empty lists. You've created one empty list, and then created a new list with three references to that same empty list. To fix the problem use this code instead:

listy = [[] for i in range(3)] 

Running your example code now gives the result you probably expected:

>>> listy = [[] for i in range(3)] >>> listy[1] = [1,2] >>> listy [[], [1, 2], []] >>> listy[1].append(3) >>> listy [[], [1, 2, 3], []] >>> listy[2].append(1) >>> listy [[], [1, 2, 3], [1]] 

Answer 2

[[]]*3 is not the same as [[], [], []].

It's as if you'd said

a = [] listy = [a, a, a] 

In other words, all three list references refer to the same list instance.