Async and Finaliser Deadlocks

A __del__ that does any kind of real work is asking for trouble. Use it to print a diagnostic reminding you to call .close() or .join() or use a with statement, and nothing else. For example:

    def close(self):
        self._closed = True
        self.do_interesting_finalisation_stuff()
    def __del__(self):
        if not self._closed:
            print("Programming error! Forgot to .close()", self)

If you do anything the slightest bit more interesting than that in your __del__, then you are likely to regret it.

Every time I've written a __del__ that did more, it has been trouble and I've ended up whittling it down to a simple diagnostic. With one notable exception: A __del__ that put a termination notification into a queue.Queue which a different thread was listening to. That one worked great: If the other thread was still alive and listening, then it would get the message. If not, then the message would just get garbage-collected with the Queue, but message would be redundant anyway, so that would be fine.