langchain/callbacks/stdout.py/StdOutCallbackHandler/on_chain_start
class StdOutCallbackHandler:    def on_chain_start(
        self, serialized: Dict[str, Any], inputs: Dict[str, Any], **kwargs: Any
    ) -> None:
        """Print out that we are entering a chain."""
        class_name = serialized["name"]
        print(f"\n\n\033[1m> Entering new {class_name} chain...\033[0m")