Funzione rfind e find

def find_string(string, find, start = None, end = None):
# @param string, find: Str.
# @param start, end: Int. Both are optional
  if start == None:
    start = 0
  if end == None:
    end = len(string)
  for posS in range(start,end):
    if string[posS] == find[0]:
      if check_find(posS, string, find):
        return posS
  return -1

def check_find(posS, string, find) :
  posS += 1
  posF = 1
  while posF < len(find):
    if not (string[posS] == find[posF]):
      return False
    posF, posS = posF+1, posS+1
  return True

##################################################################

def rfind_string(string, find, start = None, end = None):
# @param string, find: Str.
# @param start, end: Int. Both are optional
  if start == None:
    start = -1
  if end == None:
    end = len(string)-1
  for posS in range(end,start,-1):
    if string[posS] == find[len(find)-1]:
      check = check_rfind(posS, string, find)
      if check != "False":
        return check
  return -1

def check_rfind(posS, string, find) :
  F = "False"
  posF = len(find)-1
  while posF >= 0 :
    if not (string[posS] == find[posF]):
      return F
    posF, posS = posF-1, posS-1
  return posS +1