Path: csiph.com!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.xs4all.nl!newsfeed4.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Content-Type: text/plain; charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 6.6 \(1510\))
Subject: Re: Design thought for callbacks
From: Cem Karan <cfkaran2@gmail.com>
In-Reply-To: <cl2hfhFftgtU1@mid.individual.net>
Date: Tue, 24 Feb 2015 06:06:33 -0500
Content-Transfer-Encoding: quoted-printable
References: <33677AE8-B2FA-49F9-9304-C8D93784255D@gmail.com> <mailman.18947.1424497301.18130.python-list@python.org> <54e8af1b$0$12976$c3e8da3$5496439d@news.astraweb.com> <mailman.18987.1424587462.18130.python-list@python.org> <cktoi3F7hlsU1@mid.individual.net> <mailman.19016.1424614976.18130.python-list@python.org> <cl2hfhFftgtU1@mid.individual.net>
To: Gregory Ewing <greg.ewing@canterbury.ac.nz>, random832@fastmail.us
Cc: python-list@python.org
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.19123.1424775997.18130.python-list@python.org>
Lines: 50
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:86313

I'm combining two messages into one,=20

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.
>>=20
>> 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.
>=20
> 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.
>=20
> 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=