insert() but with negative index

[micseydel]

Your confusion makes sense. It took me a moment to realize it too. You're thinking that it inserts intp the index you provide. This is not quite correct.

Insert an item at a given position. The first argument is the index of the element before which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x).

Given this, the behavior makes sense: -1 is the index of the last element, so you insert before it.