Helper for session-related API calls.

def spanner_api(self):
        """Helper for session-related API calls."""
        if self._spanner_api is None:
            credentials = self._instance._client.credentials
            if isinstance(credentials, google.auth.credentials.Scoped):
                credentials = credentials.with_scopes((SPANNER_DATA_SCOPE,))
            self._spanner_api = SpannerClient(
                credentials=credentials, client_info=_CLIENT_INFO
            )
        return self._spanner_api