Path: csiph.com!usenet.pasdenom.info!aioe.org!news.stack.nl!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!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 0.003 X-Spam-Evidence: '*H*': 0.99; '*S*': 0.00; 'function,': 0.07; 'objects,': 0.07; 'api': 0.09; "'''": 0.09; 'friday,': 0.09; 'immutable': 0.09; 'objects.': 0.09; 'subject:set': 0.09; 'terms,': 0.09; 'tuple': 0.09; 'aug': 0.13; 'sat,': 0.15; 'above?': 0.16; 'auxiliary': 0.16; 'chris,': 0.16; 'encouraging.': 0.16; 'from:addr:mrabarnett.plus.com': 0.16; 'from:addr:python': 0.16; 'from:name:mrab': 0.16; 'iterators': 0.16; 'message- id:@mrabarnett.plus.com': 0.16; 'received:84.93': 0.16; 'received:84.93.230': 0.16; 'wrote:': 0.17; 'hacking': 0.17; 'url:dev': 0.17; 'hack': 0.18; 'obviously': 0.18; 'earlier': 0.21; "python's": 0.23; 'references': 0.23; 'header:In-Reply-To:1': 0.25; 'header:User-Agent:1': 0.26; '(see': 0.27; 'am,': 0.27; 'question': 0.27; 'then.': 0.27; "doesn't": 0.28; 'chris': 0.28; 'post': 0.28; 'detector': 0.29; 'received:192.168.1.3': 0.29; 'types.': 0.29; 'included': 0.29; "skip:' 10": 0.30; 'primary': 0.30; 'implement': 0.32; 'url:python': 0.32; 'received:84': 0.32; 'doubt': 0.33; 'url:home': 0.33; 'problem': 0.33; 'to:addr:python- list': 0.33; 'another': 0.33; 'changed': 0.34; 'thanks': 0.34; 'list': 0.35; 'involving': 0.35; 'nature': 0.35; 'there': 0.35; 'created': 0.36; 'but': 0.36; 'url:org': 0.36; "didn't": 0.36; 'anything': 0.36; 'possible': 0.37; 'does': 0.37; 'subject:: ': 0.38; 'object': 0.38; 'url:docs': 0.38; 'to:addr:python.org': 0.39; 'release': 0.39; 'received:192': 0.39; 'received:192.168': 0.40; 'help': 0.40; 'your': 0.60; 'between': 0.63; 'url:%20': 0.63; 'skip:n 10': 0.63; 'url:png': 0.63; 'more': 0.63; 'visit': 0.64; 'charset:windows-1252': 0.65; 'august': 0.66; 'header:Reply- To:1': 0.68; 'url:%1': 0.68; 'reply-to:no real name:2**0': 0.72; 'it\x92s': 0.84; 'premise': 0.84; 'reply-to:addr:python.org': 0.84; 'reasoning': 0.91 X-CM-Score: 0.00 X-CNFS-Analysis: v=2.0 cv=IekFqBWa c=1 sm=1 a=0nF1XD0wxitMEM03M9B4ZQ==:17 a=DKcI9XZsuF4A:10 a=ihvODaAuJD4A:10 a=OUOv7kDek9cA:10 a=N659UExz7-8A:10 a=q5VkLuigtzUA:10 a=EBOSESyhAAAA:8 a=8AHkEIZyAAAA:8 a=pGLkceISAAAA:8 a=Mx8UuZNaAAAA:20 a=fr8g_t90U3wMtSblveEA:9 a=pILNOxqGKmIA:10 a=45EfOfCuGBAA:10 a=MSl-tDqOz04A:10 a=0nF1XD0wxitMEM03M9B4ZQ==:117 X-AUTH: mrabarnett:2500 Date: Sat, 18 Aug 2012 23:14:05 +0100 From: MRAB User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:14.0) Gecko/20120713 Thunderbird/14.0 MIME-Version: 1.0 To: python-list@python.org Subject: Re: set and dict iteration References: <7xy5le7cli.fsf@ruckus.brouhaha.com> <502dab6c$0$29978$c3e8da3$5496439d@news.astraweb.com> In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 8bit X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.12 Precedence: list Reply-To: python-list@python.org 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: 45 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1345328046 news.xs4all.nl 6975 [2001:888:2000:d::a6]:37553 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:27335 On 18/08/2012 21:29, Aaron Brady wrote: > On Friday, August 17, 2012 4:57:41 PM UTC-5, Chris Angelico wrote: >> On Sat, Aug 18, 2012 at 4:37 AM, Aaron Brady wrote: >> >> > Is there a problem with hacking on the Beta? >> >> >> >> Nope. Hack on the beta, then when the release arrives, rebase your >> >> work onto it. I doubt that anything of this nature will be changed >> >> between now and then. >> >> >> >> ChrisA > > Thanks Chris, your post was encouraging. > > I have a question about involving the 'tp_clear' field of the types. > > http://docs.python.org/dev/c-api/typeobj.html#PyTypeObject.tp_clear > > ''' > ...The tuple type does not implement a tp_clear function, because it’s possible to prove that no reference cycle can be composed entirely of tuples. > ''' > > I didn't follow the reasoning in the proof; the premise is necessary but IMHO not obviously sufficient. Nevertheless, the earlier diagram contains an overt homogeneous reference cycle. > > Reposting: http://home.comcast.net/~castironpi-misc/clpy-0062%20set%20iterators.png > > In my estimate, the 'tp_traverse' and 'tp_clear' fields of the set doesn't need to visit the auxiliary collection; the same fields of the iterators don't need to visit the primary set or other iterators; and references in the linked list don't need to be included in the iterators' reference counts. > > Can someone who is more familiar with the cycle detector and cycle breaker, help prove or disprove the above? > In simple terms, when you create an immutable object it can contain only references to pre-existing objects, but in order to create a cycle you need to make an object refer to another which is created later, so it's not possible to create a cycle out of immutable objects. However, using Python's C API it _is_ possible to create such a cycle, by mutating an otherwise-immutable tuple (see PyTuple_SetItem and PyTuple_SET_ITEM).