get_chat_history = self.get_chat_history or _get_chat_history
        chat_history_str = get_chat_history(inputs["chat_history"])
        if chat_history_str:
            new_question = await self.question_generator.arun(
                question=question, chat_history=chat_history_str
            )
        else:
            new_question = question
        # TODO: This blocks the event loop, but it's not clear how to avoid it.
        docs = self._get_docs(new_question, inputs)
        new_inputs = inputs.copy()
        new_inputs["question"] = new_question
        new_inputs["chat_history"] = chat_history_str
        answer, _ = await self.combine_docs_chain.acombine_docs(docs, **new_inputs)
        if self.return_source_documents:
            return {self.output_key: answer, "source_documents": docs}
        else:
            return {self.output_key: answer}
    def save(self, file_path: Union[Path, str]) -> None:
        if self.get_chat_history:
            raise ValueError("Chain not savable when `get_chat_history` is not None.")
        super().save(file_path)
[docs]class ConversationalRetrievalChain(BaseConversationalRetrievalChain, BaseModel):
    """Chain for chatting with an index."""
    retriever: BaseRetriever
    def _get_docs(self, question: str, inputs: Dict[str, Any]) -> List[Document]:
        return self.retriever.get_relevant_documents(question)
[docs]    @classmethod
    def from_llm(
        cls,
        llm: BaseLanguageModel,
        retriever: BaseRetriever,
        condense_question_prompt: BasePromptTemplate = CONDENSE_QUESTION_PROMPT,
        qa_prompt: Optional[BasePromptTemplate] = None,