On Feb 21, 2015, at 12:42 AM, Chris Angelico <ros...@gmail.com> wrote:
> On Sat, Feb 21, 2015 at 1:44 PM, Cem Karan <cfkar...@gmail.com> wrote: >> 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? >> > > No, it's not. I would advise using strong references - if the callback > is a closure, for instance, you need to hang onto it, because there > are unlikely to be any other references to it. If I register a > callback with you, I expect it to be called; I expect, in fact, that > that *will* keep my object alive. OK, so it would violate the principle of least surprise for you. Interesting. Is this a general pattern in python? That is, callbacks are owned by what they are registered with? In the end, I want to make a library that offers as few surprises to the user as possible, and no matter how I think about callbacks, they are surprising to me. If callbacks are strongly-held, then calling 'del foo' on a callable object may not make it go away, which can lead to weird and nasty situations. Weakly-held callbacks mean that I (as the programmer), know that objects will go away after the next garbage collection (see Frank's earlier message), so I don't get 'dead' callbacks coming back from the grave to haunt me. So, what's the consensus on the list, strongly-held callbacks, or weakly-held ones? Thanks, Cem Karan -- https://mail.python.org/mailman/listinfo/python-list
