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Re: Design thought for callbacks

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On Sat, Feb 21, 2015 at 1:57 PM, Grant Edwards <invalid@invalid.invalid> wrote:
> On 2015-02-21, Cem Karan <cfkaran2@gmail.com> wrote:
>>
>> On Feb 21, 2015, at 12:42 AM, Chris Angelico <rosuav@gmail.com> wrote:
>>
>>> On Sat, Feb 21, 2015 at 1:44 PM, Cem Karan <cfkaran2@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?

Count me in the weak-ref crowd. It may be a nuisance to keep a
reference around on the object registering the callback, but it's
preferable to the alternative of messing around with disposables in
order to ensure that the callback gets cleaned up and doesn't create a
memory leak. I would also rather have my code fail by losing a
callback reference, which should be relatively easy to spot and
diagnose, than to have said memory leak go unnoticed.

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Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-21 08:13 -0500
  Re: Design thought for callbacks Grant Edwards <invalid@invalid.invalid> - 2015-02-21 20:57 +0000
    Re: Design thought for callbacks Marko Rauhamaa <marko@pacujo.net> - 2015-02-21 23:57 +0200
      Re: Design thought for callbacks Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-22 13:04 +1100
        Re: Design thought for callbacks Chris Angelico <rosuav@gmail.com> - 2015-02-22 13:11 +1100
          Re: Design thought for callbacks Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-22 15:38 +1100
            Re: Design thought for callbacks Chris Angelico <rosuav@gmail.com> - 2015-02-22 17:21 +1100
              Re: Design thought for callbacks Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-22 21:32 +1100
                Re: Design thought for callbacks Chris Angelico <rosuav@gmail.com> - 2015-02-22 22:14 +1100
                Re: Design thought for callbacks Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-23 00:45 +1100
                Re: Design thought for callbacks Chris Angelico <rosuav@gmail.com> - 2015-02-23 01:01 +1100
        Re: Design thought for callbacks Marko Rauhamaa <marko@pacujo.net> - 2015-02-22 09:52 +0200
          Re: Design thought for callbacks Chris Angelico <rosuav@gmail.com> - 2015-02-22 19:17 +1100
            Re: Design thought for callbacks Marko Rauhamaa <marko@pacujo.net> - 2015-02-22 10:34 +0200
              Re: Design thought for callbacks Chris Angelico <rosuav@gmail.com> - 2015-02-22 19:58 +1100
                Re: Design thought for callbacks Marko Rauhamaa <marko@pacujo.net> - 2015-02-22 11:14 +0200
                Re: Design thought for callbacks Chris Angelico <rosuav@gmail.com> - 2015-02-22 21:21 +1100
                Re: Design thought for callbacks Laura Creighton <lac@openend.se> - 2015-02-22 12:28 +0100
                Re: Design thought for callbacks Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-23 00:57 +1100
                Re: Design thought for callbacks Marko Rauhamaa <marko@pacujo.net> - 2015-02-22 16:06 +0200
    Re: Design thought for callbacks Ian Kelly <ian.g.kelly@gmail.com> - 2015-02-22 00:05 -0700
    Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-22 08:21 -0500
      Re: Design thought for callbacks Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-02-24 18:20 +1300
        Re: Design thought for callbacks random832@fastmail.us - 2015-02-24 00:29 -0500
          Re: Design thought for callbacks Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-02-25 10:19 +1300
            Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-25 07:50 -0500
              Re: Design thought for callbacks Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-02-26 18:36 +1300
                Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-26 05:58 -0500
                Re: Design thought for callbacks Ian Kelly <ian.g.kelly@gmail.com> - 2015-02-26 12:54 -0700
                Re: Design thought for callbacks Ethan Furman <ethan@stoneleaf.us> - 2015-02-26 12:00 -0800
                Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-03-02 06:06 -0500
                Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-03-02 06:04 -0500
                Re: Design thought for callbacks Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-02 08:34 -0700
                Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-03-08 14:30 -0400

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