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To: python-list@python.org
From: "Frank Millman" <frank@chagford.com>
Subject: Re: Design thought for callbacks
Date: Sat, 21 Feb 2015 07:41:14 +0200
References: <33677AE8-B2FA-49F9-9304-C8D93784255D@gmail.com>
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"Cem Karan" <cfkaran2@gmail.com> wrote in message 
news:33677AE8-B2FA-49F9-9304-C8D93784255D@gmail.com...
> Hi all, I'm working on a project that will involve the use of callbacks, 
> and I want to bounce an idea I had off of everyone to make sure I'm not 
> developing a bad idea.  Note that this is for python 3.4 code; I don't 
> need to worry about any version of python earlier than that.
>
> In order to inform users that certain bits of state have changed, I 
> require them to register a callback with my code.  The problem is that 
> when I store these callbacks, it naturally creates a strong reference to 
> the objects, which means that if they are deleted without unregistering 
> themselves first, my code will keep the callbacks alive.  Since this could 
> lead to really weird and nasty situations, I would like to store all the 
> callbacks in a WeakSet 
> (https://docs.python.org/3/library/weakref.html#weakref.WeakSet).  That 
> way, my code isn't the reason why the objects are kept alive, and if they 
> are no longer alive, they are automatically removed from the WeakSet, 
> preventing me from accidentally calling them when they are dead.  My 
> question is simple; is this a good design?  If not, why not?
>   Are there any potential 'gotchas' I should be worried about?
>

I tried something similar a while ago, and I did find a gotcha.

The problem lies in this phrase - "if they are no longer alive, they are 
automatically removed from the WeakSet, preventing me from accidentally 
calling them when they are dead."

I found that the reference was not removed immediately, but was waiting to 
be garbage collected. During that window, I could call the callback, which 
resulted in an error.

There may have been a simple workaround. Perhaps someone else can comment.

Frank Millman