def format_number(num):
  #
  #  Is the string the correct length?
  #
  if len(num) != 10:
    return ""

  #
  #  Are there any characters that aren't digits?
  #
  for ch in num:
    if ch < "0" or ch > "9":
      return ""

  #
  #  Use string slices to get the portions of the number that we need
  #
  return "(" + num[0:3] + ") " + num[3:6] + "-" + num[6:10]