On Feb 21, 2015, at 12:41 AM, Frank Millman <fr...@chagford.com> 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. THAT would be one heck of a gotcha! Must have been fun debugging that one! Cem Karan -- https://mail.python.org/mailman/listinfo/python-list
