Python reverse-stride slicing
Simply exclude the end range index...
>>> foo[3::-1]
'3210'
Ironically, about the only option I think you didn't try.
Omit the end index in your slice notation:
>>> foo = '0123456'
>>> foo[3::-1]
'3210'
If you have to do this many times, create a slice object that you can use over and over
>>> i = slice(3,None,-1)
>>> foo[i]
'3210'