 Python has array index and array slicing expressions on lists , denoted as a [ key ] , a [ start : stop ] or a [ start : stop : step ] . Indexes are zero @-@ based , and negative indexes are relative to the end . Slices take elements from the start index up to , but not including , the stop index . The third slice parameter , called step or stride , allows elements to be skipped and reversed . Slice indexes may be omitted , for example a [ : ] returns a copy of the entire list . Each element of a slice is a shallow copy . 
