langchain/chains/conversational_retrieval/base.py/BaseConversationalRetrievalChain/output_keys
class BaseConversationalRetrievalChain:    def output_keys(self) -> List[str]:
        """Return the output keys.

        :meta private:
        """
        _output_keys = [self.output_key]
        if self.return_source_documents:
            _output_keys = _output_keys + ["source_documents"]
        return _output_keys