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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)