Design thought for callbacks

[Cem Karan]

On Feb 21, 2015, at 8:37 AM, Mark Lawrence <breamoreboy at yahoo.co.uk> wrote:

> 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.


Unfortunately, depending on how many objects you have in your object graph, it can slow your code down a fair amount.  I think Frank is right about how a WeakSet might be a bad idea in this case.  You really need to know if an object is alive or dead, and not some indeterminate state.

Thanks,
Cem Karan