Rfind

def Rfind(s,sub_string,start = 0, stop = None):
#@param s: string; stringa su cui cercare
#@param sub_string: string; stringa da cercare
#@param start: int
#@param stop: int

    if stop == None or stop >= len(s):
        stop = len(s)-1

    if start>=len(s) and stop >=len(s):
        return -1

    if start==stop:
        return -1

    lensub = len(sub_string)

    if(stop-start)+1>=lensub:
        if lensub == 1:
            for i in range(stop,start-1,-1):
                if s[i] == sub_string:
                    return i

            return -1

        else:
            for i in range(stop,start-1,-1):
                if (i-lensub+1)>=start:
                    if s[i-lensub+1:i+1] == sub_string:
                        return i-lensub+1
                else:
                    return -1

            return -1

    else:
        return -1