code: def normalize_digits_only(number, keep_non_digits=False):
      
      number = unicod(number)
      number_length = len(number)
      normalized_digits = U_EMPTY_STRING
      for ii in range(number_length):
          d = unicode_digit(number[ii], -1)
          if d != -1:
              normalized_digits += unicod(d)
          elif keep_non_digits:
              normalized_digits += number[ii]
      return normalized_digits
docs: Normalizes a string of characters representing a phone number.
  
      This converts wide-ascii and arabic-indic numerals to European numerals,
      and strips punctuation and alpha characters (optional).
  
      Arguments:
      number -- a string representing a phone number
      keep_non_digits -- whether to keep non-digits
  
      Returns the normalized string version of the phone number.