langchain/chains/qa_with_sources/base.py/BaseQAWithSourcesChain/output_keys
class BaseQAWithSourcesChain:    def output_keys(self) -> List[str]:
        """Return output key.

        :meta private:
        """
        _output_keys = [self.answer_key, self.sources_answer_key]
        if self.return_source_documents:
            _output_keys = _output_keys + ["source_documents"]
        return _output_keys