code: def spanner_api(self):
          
          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
docs: Helper for session-related API calls.