Say you have a list with the numbers 0 - 99. You are writing a program that takes the last item in the list and uses it for some other purpose. Is it more efficient to use list[-1] than to use list[99]? In other words, does Python iterate through the whole list in either case?
Python does not iterate through lists to find a particular index. Lists are arrays (of pointers to elements) in contiguous memory and so locating the desired element is always a simple multiplication and addition. If anything, list[-1] will be slightly slower because Python needs to add the negative index to the length to get the real index. (I doubt it is noticeably slower, however, because all that's done in C anyway.)