Python: Slicing in reverse, in the middle of a sequence

When slicing forwards, it's relatively simple to understand: s[7:9] returns a 2-item sequence of elements 7 and 8.  This works pretty much like any other half-open interval, in which one side (the 7) is included and the other (the 9) excluded.  The resulting length is simply the difference between the end and start indexes, 9-7=2.

What about backward? If you reverse the numbers and add a stride value, s[9:7:-1] gives you elements 9 and 8.  Since the interval is still half-open, now 9 is on the closed end and included, and 7 is open and excluded.  So s[8:6:-1] is the reverse of s[7:9].  You're getting two elements, starting at 8 and ending before 6, going backwards.

What happens if you want to get the reverse of s[0:5]?  The above math would suggest s[4:-1:-1] but negative indexes are way at the other end of the sequence, so this produces an empty result.  The correct answer is actually omitting the end index, as in s[4::-1].  That invokes the regular "all items remaining in sequence" meaning, that is also used in s[9:].