docs: def _check_not_empty(string):
      """
      Checks that the string is not empty.
  
      If it is empty an exception is raised, stopping the validation.
  
      This is used for compulsory alphanumeric fields.
  
      :param string: the field value
      """
      string = string.strip()
  
      if len(string) == 0:
          message = 'The string should not be empty'
          raise pp.ParseException(message)