docs: def _flush(self):
          """
          Flush the write buffers of the stream.
          """
          # Initialize multi-part upload
          if 'UploadId' not in self._upload_args:
              with _handle_client_error():
                  self._upload_args[
                      'UploadId'] = self._client.create_multipart_upload(
                      **self._client_kwargs)['UploadId']
  
          # Upload part with workers
          response = self._workers.submit(
              self._client.upload_part, Body=self._get_buffer().tobytes(),
              PartNumber=self._seek, **self._upload_args)
  
          # Save part information
          self._write_futures.append(
              dict(response=response, PartNumber=self._seek))