Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder.erje.net!newsfeed.xs4all.nl!newsfeed5.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 0.005 X-Spam-Evidence: '*H*': 0.99; '*S*': 0.00; 'raised': 0.07; '(it': 0.09; 'raised,': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:80.91.229.12': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'received:lo.gmane.org': 0.09; 'underlying': 0.09; 'subject:error': 0.09; 'am,': 0.12; '(call': 0.16; 'enigma': 0.16; 'iterator': 0.16; 'iterator.': 0.16; 'kern': 0.16; 'swallow': 0.16; 'unpacking': 0.16; 'valueerror': 0.16; 'wrote:': 0.18; 'dec': 0.22; 'header:In-Reply-To:1': 0.22; 'values.': 0.23; 'times,': 0.24; 'all,': 0.28; 'raise': 0.28; "wasn't": 0.28; 'interpret': 0.29; 'temporary': 0.29; 'pm,': 0.29; 'chris': 0.30; 'values': 0.32; 'list': 0.32; 'header:User-Agent:1': 0.33; 'header:X-Complaints-To:1': 0.33; 'there': 0.33; 'object': 0.33; 'fri,': 0.34; 'to:addr:python-list': 0.34; 'too': 0.34; 'but': 0.37; 'could': 0.37; 'received:org': 0.38; 'should': 0.39; 'to:addr:python.org': 0.40; 'more': 0.61; '2011': 0.61; 'back': 0.62; 'world': 0.62; 'received:86': 0.63; 'results': 0.63; 'our': 0.64; 'fall': 0.64; 'believe': 0.65; 'roughly': 0.67; 'subject:day': 0.68; 'informative': 0.73; 'andrea': 0.84; 'yielded': 0.84; 'eco': 0.91 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Robert Kern Subject: Re: Misleading error message of the day Date: Thu, 08 Dec 2011 19:57:15 +0000 References: <18647617.2258.1323358966076.JavaMail.geo-discussion-forums@yqf20> <4EE0DDE4.4080802@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: cpc24-cmbg15-2-0-cust204.5-4.cable.virginmedia.com User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:8.0) Gecko/20111105 Thunderbird/8.0 In-Reply-To: X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.12 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: 31 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1323374249 news.xs4all.nl 6941 [2001:888:2000:d::a6]:37553 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:16868 On 12/8/11 4:21 PM, Chris Angelico wrote: > On Fri, Dec 9, 2011 at 2:55 AM, Andrea Crotti wrote: >> Yes but how do you know how many values you generated when it quits? >> I mean I don't know how it work internally, but it should keep a temporary >> list of the yielded values to be able to find out how many values are >> there.. > > Iterator unpacking works roughly thus: > > 1) Count up how many results you need (call that N) > 2) N times, get a value from the iterator. If StopIteration is raised, > swallow it and raise ValueError because there were too few values. > 3) Attempt to get one more value from the iterator. If StopIteration > is NOT raised, raise ValueError because there were too many values. > > At no point is the "total size" of the iterator counted (it could, > after all, be infinite). When ValueError is raised, all that's known > is that StopIteration wasn't raised at the end of the process. unpack_iterable() has the original object available to it, not just the iterator. It could opportunistically check for __len__() and fall back to the less informative message when it is absent. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco