langchain/callbacks/streaming_stdout_final_only.py/FinalStreamingStdOutCallbackHandler/__init__
class FinalStreamingStdOutCallbackHandler:    def __init__(self, answer_prefix_tokens: Optional[List[str]] = None) -> None:
        super().__init__()
        if answer_prefix_tokens is None:
            answer_prefix_tokens = DEFAULT_ANSWER_PREFIX_TOKENS
        self.answer_prefix_tokens = answer_prefix_tokens
        self.last_tokens = [""] * len(answer_prefix_tokens)
        self.answer_reached = False