def METHOD_NAME(self, exception, handler, route_names: Optional[List[str]] = None):
    """Add a new exception handler to an already existing handler object.
    Args:
        exception (sanic.exceptions.SanicException or Exception): Type
            of exception that needs to be handled.
        handler (function): Reference to the function that will
            handle the exception.
    Returns:
        None
    """  # noqa: E501
    if route_names:
        for route in route_names:
            self._add((exception, route), handler)
    else:
        self._add((exception, None), handler)