How to explain the reverse of a sequence by slice notation a[::-1]

Question

From the python.org tutorial

Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size

Answer 1

You'll notice that the third slice argument, the step, is not presented in the part of the tutorial you quoted. That particular snippet assumes a positive step.

When you add in the possibility of a negative step, the behavior is actually pretty intuitive. An empty start parameter refers to whichever end of the sequence one would start at to step through the whole sequence in the direction indicated by the step value. In other words it refers to the lowest index (to count up) if you have a positive step, and the highest index (to count down) if you have a negative step. Likewise, an empty end parameter refers to whichever end of the sequence one would end up at after stepping through in the appropriate direction.