The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that 0 <= n < (j-i)/k. In other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j). If i or j is greater than len(s), use len(s). If i or j are omitted or None, they become “end” values (which end depends on the sign of k). Note, k cannot be zero. If k is None, it is treated like 1.
оно не работает! ибо «идти» нужно с конца последовательности, т.е. рабочий пример должен быть таким:
кстати, стоит подумать дважды, чтобы понять, что будет при отрицательном step
docs.python.org/2/library/stdtypes.html