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Re: Design thought for callbacks

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I'm combining two messages into one, 

On Feb 24, 2015, at 12:29 AM, random832@fastmail.us wrote:

> On Tue, Feb 24, 2015, at 00:20, Gregory Ewing wrote:
>> Cem Karan wrote:
>>> I tend to structure my code as a tree or DAG of objects.  The owner refers to
>>> the owned object, but the owned object has no reference to its owner.  With
>>> callbacks, you get cycles, where the owned owns the owner.
>> 
>> This is why I suggested registering a listener object
>> plus a method name instead of a callback. It avoids that
>> reference cycle, because there is no long-lived callback
>> object keeping a reference to the listener.
> 
> How does that help? Everywhere you would have had a reference to the
> "callback object", you now have a reference to the listener object.
> You're just shuffling deck chairs around: if B shouldn't reference A
> because A owns B, then removing C from the B->C->A reference chain does
> nothing to fix this.

On Feb 24, 2015, at 12:45 AM, Gregory Ewing <greg.ewing@canterbury.ac.nz> wrote:

> Cem Karan wrote:
>> On Feb 22, 2015, at 5:15 AM, Gregory Ewing <greg.ewing@canterbury.ac.nz>
>> wrote:
>>> Perhaps instead of registering a callback function, you should be
>>> registering the listener object together with a method name.
>> I see what you're saying, but I don't think it gains us too much.  If I store
>> an object and an unbound method of the object, or if I store the bound method
>> directly, I suspect it will yield approximately the same results.
> 
> It would be weird and unpythonic to have to register both
> an object and an unbound method, and if you use a bound
> method you can't keep a weak reference to it.


Greg, random832 said what I was thinking earlier, that you've only increased the diameter of your cycle without actually fixing it.  Can you give a code example where your method breaks the cycle entirely?

Thanks,
Cem Karan

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Re: Design thought for callbacks "Frank Millman" <frank@chagford.com> - 2015-02-21 07:41 +0200
  Re: Design thought for callbacks Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-22 03:15 +1100
    Re: Design thought for callbacks Marko Rauhamaa <marko@pacujo.net> - 2015-02-21 19:08 +0200
      Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-22 07:16 -0500
        Re: Design thought for callbacks Marko Rauhamaa <marko@pacujo.net> - 2015-02-22 14:46 +0200
          Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-22 09:17 -0500
      Re: Design thought for callbacks Laura Creighton <lac@openend.se> - 2015-02-22 13:52 +0100
      Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-22 09:10 -0500
    Re: Design thought for callbacks "Frank Millman" <frank@chagford.com> - 2015-02-22 08:44 +0200
      Re: Design thought for callbacks Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-02-22 23:15 +1300
        Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-22 09:22 -0500
          Re: Design thought for callbacks Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-02-24 18:45 +1300
            Re: Design thought for callbacks Cem Karan <cfkaran2@gmail.com> - 2015-02-24 06:06 -0500
        Re: Design thought for callbacks Ian Kelly <ian.g.kelly@gmail.com> - 2015-02-22 23:57 -0700

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