from time import process_time as clock from math import floor def reverse_list(list): count = floor(len(list) / 2) for i in range(count): last_index = len(list) - i - 1 list[i], list[last_index] = list[last_index], list[i] return list start = clock() N = 20000000 l = list(range(N)) reverse_list(l) print(clock() - start, 's')