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

def _exclude_files_parser(cls, key):
        """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
        """
        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