def METHOD_NAME(
    self,
    app: AppT,
    message: Message,
    *,
    loads: Optional[Callable] = None,
    serializer: CodecArg = None,
) -> KT:
    if loads is None:
        loads = app.serializers.METHOD_NAME
    return cast(
        KT,
        loads(
            self.key_type,
            message.key,
            serializer=serializer or self.key_serializer,
        ),
    )