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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?
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Use the append() Function to Append Values to a 2D Array in Python. In this case, we will use Lists in place of arrays. The list is one of the four built-in datatypes provided in Python and is very similar to arrays. NumPy arrays can be converted to a list first using the tolist() function.
append() adds a list inside of a list. Lists are objects, and when you use . append() to add another list into a list, the new items will be added as a single object (item).
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]] [[]]*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.
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