def _parse_engine(self):
        """
        Parse the storage engine in the config.

        Returns:
            str
        """
        if self._parser.has_option('storage', 'engine'):
            engine = str(self._parser.get('storage', 'engine'))
        else:
            engine = ENGINE_DROPBOX

        assert isinstance(engine, str)

        if engine not in [ENGINE_DROPBOX,
                          ENGINE_GDRIVE,
                          ENGINE_COPY,
                          ENGINE_ICLOUD,
                          ENGINE_BOX,
                          ENGINE_FS]:
            raise ConfigError('Unknown storage engine: {}'.format(engine))

        return str(engine)
Parse the storage engine in the config.

        Returns:
            str