langchain/callbacks/base.py/BaseCallbackManager/__init__
class BaseCallbackManager:    def __init__(
        self,
        handlers: List[BaseCallbackHandler],
        inheritable_handlers: Optional[List[BaseCallbackHandler]] = None,
        parent_run_id: Optional[UUID] = None,
    ) -> None:
        """Initialize callback manager."""
        self.handlers: List[BaseCallbackHandler] = handlers
        self.inheritable_handlers: List[BaseCallbackHandler] = (
            inheritable_handlers or []
        )
        self.parent_run_id: Optional[UUID] = parent_run_id