TypeError: ord() expected a character,but string of lenth 4 found

# -*- coding: utf-8 -*-
# @Date : 2018-03-01 12:51:08
# @Author : Termal (Termal@example.org)
# @Link : http://Thermal.org
#功能：比较a，b两个字符串是否相同
def comparison(a,b):
 ib=0
 for ia in range(len(a)):
  if ord(a[ia:ia+1])-ord(b[ib:ib+1])==0:
   ib=ib+1
   if ib==len(b):
    print('a and b are equall')
  else:
   print('a and b are not equall')
   break