On 2015-02-21, Cem Karan <cfkar...@gmail.com> wrote: > > 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.
And me as well. I would expect to be able to pass a closure as a callback and not have to keep a reference to it. Perhaps that just a leftover from working with other languages (javascript, scheme, etc.). It doesn't matter if it's a string, a float, a callback, a graphic or whatever: if I pass your function/library an object, I expect _you_ to keep track of it until you're done with it. > Interesting. Is this a general pattern in python? That is, > callbacks are owned by what they are registered with? I'm not sure what you mean by "owned" or why it matters that it's a callback: it's an object that was passed to you: you need to hold onto a reference to it until you're done with it, and the polite thing to do is to delete references to it when you're done with it. > So, what's the consensus on the list, strongly-held callbacks, or > weakly-held ones? -- Grant -- https://mail.python.org/mailman/listinfo/python-list
