Adding a list to an empty python list issue

Question

I am Python beginner with no prior programming experience. I apologize in advance if this question sounds too lame to answer to, but there is something I am confused with. It's related to adding a particular list (l2) to an empty list (l1). What I am trying to do is to increase the value of each of the "l2" elements in each iteration, and then add it to the l2 list.Here is the code:

l1 = []
l2 = [0,1,2,3]

for i in range(3):
    l2[0] = l2[0] + 1
    l2[1] = l2[1] + 1
    l2[2] = l2[2] + 1
    l2[3] = l2[3] + 1
    print l2
    l1.append(l2)

print l1

This is the print result that I get:

[1, 2, 3, 4]
[2, 3, 4, 5]
[3, 4, 5, 6]
[[3, 4, 5, 6], [3, 4, 5, 6], [3, 4, 5, 6]]

Why doesn't l2 list look like this:

[[1, 2, 3, 4], [2, 3, 4, 5], [3, 4, 5, 6]]

? Thank you for the reply.

Answer 1

It is because you are appending the same list object each time. You need to copy the list object before you append it, so that you are sure you append a different object.

l1.append(l2[:])