I've seen that ::-1 can be used to reverse a string, but I thought it would be of interest to find the explicit integers i:j:k that would do the job.
I found the documentation common-sequence-operations, but can't get it to work. This code test comes up short:
s = "abcd"
print(1,s[4:3:-1])
print(2,s[4:2:-1])
print(3,s[4:1:-1])
print(4,s[4:0:-1])
print(5,s[4:-1:-1])
Here is the output:
1
2 d
3 dc
4 dcb
5
It looks like you can't do this with an explicit formula, s[i:j:k].
Usually I can figure out simple programming by trial-n-error, but that is not working here. Maybe a careful read of the documentation will get me over this!
You would have to do it like this:
s = "abcd"
print(s[len(s):-len(s)-1:-1])
Or, as pointed out by Terry Jan Reedy in the comments, the following way of writing it illustrates nicely the length of the slice as (stop - start) / step:
print(s[-1:-1-len(s):-1])
The issue is that negative indexing starts at the back, so the negative indexes correspond the following "real" indexes:
-1 -> 3
-2 -> 2
-3 -> 1
-4 -> 0 # -len(s)
-5 -> -1 # that's the one you need (or any smaller number)
0 or -len(s) is not enough since the stop index of the slice is exclusive, so you have to get one lower.