Design thought for callbacks

[Cem Karan]

On Feb 22, 2015, at 7:52 AM, Laura Creighton <lac at openend.se> wrote:

> In a message of Sun, 22 Feb 2015 07:16:14 -0500, Cem Karan writes:
> 
>> This was PRECISELY the situation I was thinking about.  My hope was
>> to make the callback mechanism slightly less surprising by allowing
>> the user to track them, releasing them when they aren't needed
>> without having to figure out where the callbacks were registered.
>> However, it appears I'm making things more surprising rather than
>> less.
> 
> You may be able to accomplish your goal by using a Queue with a
> producer/consumer model.
> see: http://stackoverflow.com/questions/9968592/turn-functions-with-a-callback-into-python-generators
> 
> especially the bottom of that.
> 
> I haven't run the code, but it looks mostly reasonable, except that
> you do not want to rely on the Queue maxsize being 1 here, and
> indeed, I almost always want a bigger Queue  in any case.  Use
> Queue.task_done if blocking the producer features in your design.
> 
> The problem that you are up against is that callbacks are inherantly
> confusing, even to programmers who are learning about them for the
> first time.  They don't fit people's internal model of 'how code works'.
> There isn't a whole lot one can do about that except to
> try to make the magic do as little as possible, so that more of the
> code works 'the way people expect'.

I think what you're suggesting is that library users register a Queue instead of a callback, correct?  The problem is that I'll then have a strong reference to the Queue, which means I'll be pumping events into it after the user code has gone away.  I was hoping to solve the problem of forgotten registrations in the library.

Thanks,
Cem Karan