def _open_for_read(self):
        """open the file in read mode"""
        ownerid, datasetid = parse_dataset_key(self._dataset_key)
        response = requests.get(
            '{}/file_download/{}/{}/{}'.format(
                self._query_host, ownerid, datasetid, self._file_name),
            headers={
                'User-Agent': self._user_agent,
                'Authorization': 'Bearer {}'.format(
                    self._config.auth_token)
            }, stream=True)
        try:
            response.raise_for_status()
        except Exception as e:
            raise RestApiError(cause=e)
        self._read_response = response
open the file in read mode