On Feb 21, 2015, at 8:37 AM, Mark Lawrence <breamore...@yahoo.co.uk> wrote:
> On 21/02/2015 05:41, Frank Millman wrote: >> >> "Cem Karan" <cfkar...@gmail.com> wrote in message >> news:33677ae8-b2fa-49f9-9304-c8d937842...@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 -- https://mail.python.org/mailman/listinfo/python-list
