[python] Appending to 2D lists in Python

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?

[[]]*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.

Came here to see how to append an item to a 2D array, but the title of the thread is a bit misleading because it is exploring an issue with the appending.

The easiest way I found to append to a 2D list is like this:

list=[[]]

list.append((var_1,var_2))

This will result in an entry with the 2 variables var_1, var_2. Hope this helps!