Design thought for callbacks

[Cem Karan]

On Feb 21, 2015, at 12:41 AM, Frank Millman <frank at chagford.com> wrote:

> 
> "Cem Karan" <cfkaran2 at gmail.com> wrote in message 
> news:33677AE8-B2FA-49F9-9304-C8D93784255D at 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.

THAT would be one heck of a gotcha!  Must have been fun debugging that one!

Cem Karan