code: def valid(number):
      
      checksum = 0
  
      number_len = len(number)
      offset = ord('0')
  
      i = number_len - 1
      while i >= 0:
        n = ord(number[i]) - offset
        checksum += n
        i -= 2
  
      i = number_len - 2
      while i >= 0:
        n = ord(number[i]) - offset
        n *= 2
        if n > 9:
            n -= 9
        checksum += n
        i -= 2
  
      return checksum%10 == 0
docs: Returns true if the number string is luhn valid, and false otherwise.  The
      number string passed to the function must contain only numeric characters
      otherwise behavior is undefined.