Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.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 Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.008 X-Spam-Evidence: '*H*': 0.98; '*S*': 0.00; '21,': 0.07; 'nasty': 0.07; 'bits': 0.09; 'callback': 0.09; 'falls': 0.09; 'naturally': 0.09; 'objects,': 0.09; 'url:github': 0.09; 'cc:addr:python-list': 0.11; 'python': 0.11; 'creates': 0.14; '(note': 0.16; 'design?': 0.16; 'fabio': 0.16; 'thread,': 0.16; 'top-level': 0.16; 'url:py': 0.16; 'sat,': 0.16; 'weird': 0.16; 'url:)': 0.16; 'thanks,': 0.17; 'wrote:': 0.18; 'code.': 0.18; 'all,': 0.19; '(where': 0.19; 'not,': 0.20; 'feb': 0.22; 'email addr:gmail.com>': 0.22; 'cc:addr:python.org': 0.22; 'earlier': 0.24; 'question': 0.24; 'cc:2**0': 0.24; 'first,': 0.26; 'certain': 0.27; 'developing': 0.27; 'header:In-Reply-To:1': 0.27; 'idea': 0.28; 'point': 0.28; 'am,': 0.29; 'related': 0.29; 'message-id:@mail.gmail.com': 0.30; "i'm": 0.30; 'url:mailman': 0.30; 'code': 0.31; 'that.': 0.31; 'usually': 0.31; 'accidentally': 0.31; 'themselves': 0.32; 'url:python': 0.33; 'everyone': 0.33; "i'd": 0.34; 'could': 0.34; 'problem': 0.35; 'created': 0.35; 'objects': 0.35; 'test': 0.35; 'but': 0.35; 'received:google.com': 0.35; 'there': 0.35; 'version': 0.36; 'really': 0.36; 'url:listinfo': 0.36; "didn't": 0.36; 'url:org': 0.36; 'should': 0.36; 'project': 0.37; 'url:library': 0.38; 'bad': 0.39; 'sure': 0.39; 'users': 0.40; 'url:mail': 0.40; 'how': 0.40; 'skip:u 10': 0.60; 'read': 0.60; 'worry': 0.60; 'url:3': 0.61; 'simple': 0.61; 'back': 0.62; 'kept': 0.65; 'to:addr:gmail.com': 0.65; 'subject:Design': 0.78; 'inform': 0.78; '2015': 0.84; '3.4': 0.84; 'about?': 0.84; 'preventing': 0.84; 'situations,': 0.84; 'subject:thought': 0.84; 'url:master': 0.84; 'bounce': 0.91 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc:content-type; bh=l9JtGvX/q5pbMh2W3GSaoi+3XrfhNK0TDocn2k9+RqI=; b=Gb0RUEiKP7cZkfUxvDP8/dU0VYE9C+8qCm0Udoi/93SWgtt5UQtCDxFWIu7+WsiN/K XQT1J0dDOu8Dyr37LYGMWgDUSja/8lBODWLiFCChUtSSicEwXSdq63a7VVCjkEvgs1ZG Ord072JpiRJzvtDKMWUyIXuaifAjFIbkaIYbQhv/201dIvUHy+NdpOXiZZnzWNH8D7Kd XHbLFVg6jxI7VmieCCE7rLmrlOrLsYqA4I+mojnNaw+jKla7nTje/UCH+i8+LRVOAFgZ yGqg2hh/mxUY4MqbqhBwxyQqM6uG6JNELDQGuwo+F6++7mNbxgtC5FYCrSpn8KN9nuBd NPEQ== X-Received: by 10.202.221.84 with SMTP id u81mr10564101oig.71.1424784213621; Tue, 24 Feb 2015 05:23:33 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <33677AE8-B2FA-49F9-9304-C8D93784255D@gmail.com> References: <33677AE8-B2FA-49F9-9304-C8D93784255D@gmail.com> From: Fabio Zadrozny Date: Tue, 24 Feb 2015 10:23:13 -0300 Subject: Re: Design thought for callbacks To: Cem Karan Content-Type: multipart/alternative; boundary=001a113ce330ef3ecb050fd56c38 Cc: "comp.lang.python" X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 108 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1424785683 news.xs4all.nl 2947 [2001:888:2000:d::a6]:44044 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:86318 --001a113ce330ef3ecb050fd56c38 Content-Type: text/plain; charset=UTF-8 Hi Cem, I didn't read the whole long thread, but I thought I'd point you to what I'm using in PyVmMonitor (http://www.pyvmmonitor.com/) -- which may already cover your use-case. Take a look at the callback.py at https://github.com/fabioz/pyvmmonitor-core/blob/master/pyvmmonitor_core/callback.py And its related test (where you can see how to use it): https://github.com/fabioz/pyvmmonitor-core/blob/master/_pyvmmonitor_core_tests/test_callback.py (note that it falls back to a strong reference on simple functions -- i.e.: usually top-level methods or methods created inside a scope -- but otherwise uses weak references). Best Regards, Fabio On Sat, Feb 21, 2015 at 12:44 AM, Cem Karan wrote: > Hi all, I'm working on a project that will involve the use of callbacks, > and I want to bounce an idea I had off of everyone to make sure I'm not > developing a bad idea. Note that this is for python 3.4 code; I don't need > to worry about any version of python earlier than that. > > 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? > > Thanks, > Cem Karan > -- > https://mail.python.org/mailman/listinfo/python-list > --001a113ce330ef3ecb050fd56c38 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Hi= Cem,

I didn't read= the whole long thread, but I thought I'd point you to what I'm usi= ng in PyVmMonitor (http://www.pyvmm= onitor.com/) -- which may already cover your use-case.

Take a look at the callback.py at=C2= =A0https://github.com/fabioz/pyvmmonitor-core/blob/mas= ter/pyvmmonitor_core/callback.py

And its related test (where you can see how to use it):=C2=A0= https://github.com/fabioz/pyvmmonitor-core= /blob/master/_pyvmmonitor_core_tests/test_callback.py (note that it fal= ls back to a strong reference on simple functions -- i.e.: usually top-leve= l methods or methods created inside a scope -- but otherwise uses weak refe= rences).

<= /div>
Best Regards,<= /div>

Fabio

On Sat, Feb 21, 2015 at= 12:44 AM, Cem Karan <cfkaran2@gmail.com> wrote:
Hi all, I'm working on a project that will invo= lve the use of callbacks, and I want to bounce an idea I had off of everyon= e to make sure I'm not developing a bad idea.=C2=A0 Note that this is f= or python 3.4 code; I don't need to worry about any version of python e= arlier than that.

In order to inform users that certain bits of state have changed, I require= them to register a callback with my code.=C2=A0 The problem is that when I= store these callbacks, it naturally creates a strong reference to the obje= cts, which means that if they are deleted without unregistering themselves = first, my code will keep the callbacks alive.=C2=A0 Since this could lead t= o really weird and nasty situations, I would like to store all the callback= s in a WeakSet (https://docs.python.org/3/library/weakref= .html#weakref.WeakSet).=C2=A0 That way, my code isn't the reason wh= y the objects are kept alive, and if they are no longer alive, they are aut= omatically removed from the WeakSet, preventing me from accidentally callin= g them when they are dead.=C2=A0 My question is simple; is this a good desi= gn?=C2=A0 If not, why not?=C2=A0 Are there any potential 'gotchas' = I should be worried about?

Thanks,
Cem Karan
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