Design thought for callbacks

[Mark Lawrence]

On 21/02/2015 05:41, Frank Millman 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.
>
> Frank Millman
>

https://docs.python.org/3/library/gc.html has a collect function.  That 
seems like a simple workaround, but whether or not it classifies as a 
good solution I'll leave to others, I'm not qualified to say.


-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence