code: def _parse_engine(self):
          
          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)
docs: Parse the storage engine in the config.
  
          Returns:
              str