It makes a copy of "key". It's the same as key[::]; when dealing with lists (or other mutable types) it prevents alterations affectiong the original list, as far as I know this isn't a problem with the immutable strings in python so it's probably just habit.

I've tested a couple of solutions, and this one was surprisingly fast, as long as the length of the consecutive count remained below 1000. After that, its apparent O(n^2) complexity really becomes noticable.

it does not matter, because the test case sorts our results using sorted() method which returns a list.