"""Return output keys.
        :meta private:
        """
        return self.output_variables
    def _call(
        self,
        inputs: Dict[str, str],
        run_manager: Optional[CallbackManagerForChainRun] = None,
    ) -> Dict[str, str]:
        return self.transform_cb(inputs)
    async def _acall(
        self,
        inputs: Dict[str, Any],
        run_manager: Optional[AsyncCallbackManagerForChainRun] = None,
    ) -> Dict[str, Any]:
        if self.atransform_cb is not None:
            return await self.atransform_cb(inputs)
        else:
            self._log_once(
                "TransformChain's atransform is not provided, falling"
                " back to synchronous transform"
            )
            return self.transform_cb(inputs)