code: def _next_seg(self):
      
      if self._seg:
        self._seg.close()
      self._seg_index += 1
      if self._seg_index > self._last_seg_index:
        self._seg = None
        return
  
      filename = self._seg_prefix + str(self._seg_index)
      stat = cloudstorage.stat(filename)
      writer = output_writers._GoogleCloudStorageOutputWriter
      if writer._VALID_LENGTH not in stat.metadata:
        raise ValueError(
            "Expect %s in metadata for file %s." %
            (writer._VALID_LENGTH, filename))
      self._seg_valid_length = int(stat.metadata[writer._VALID_LENGTH])
      if self._seg_valid_length > stat.st_size:
        raise ValueError(
            "Valid length %s is too big for file %s of length %s" %
            (self._seg_valid_length, filename, stat.st_size))
      self._seg = cloudstorage.open(filename)
docs: Get next seg.