code: def addinterval(instr, add, interval):
      '''
          adds string every n character. returns string
      '''
      if not isinstance(instr, str):
          instr = str(instr)
  
      return add.join(
          instr[i:i+interval]
          for i in xrange(0,len(instr),interval)f addinterval(instr, add, interval):
      '''
          adds string every n character. returns string
      '''
      if not isinstance(instr, str):
          instr = str(instr)
  
      return add.join(
          instr[i:i+interval]
          for i in xrange(0,len(instr),interval))
docs: adds string every n character. returns string