code: def extract(self, path, extract_method, to_path):
      
      self._pbar_path.update_total(1)
      if extract_method not in _EXTRACT_METHODS:
        raise ValueError('Unknown extraction method "%s".' % extract_method)
      future = self._executor.submit(self._sync_extract,
                                     path, extract_method, to_path)
      return promise.Promise.resolve(future)
docs: Returns `promise.Promise` => to_path.