"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://mail.python.org/mailman/listinfo/python-list
