code: def _exclude_files_parser(cls, key):
          
          def parser(value):
              exclude_directive = 'file:'
              if value.startswith(exclude_directive):
                  raise ValueError(
                      'Only strings are accepted for the {0} field, '
                      'files are not accepted'.format(key))
              return value
          return parser
docs: Returns a parser function to make sure field inputs
          are not files.
  
          Parses a value after getting the key so error messages are
          more informative.
  
          :param key:
          :rtype: callable