Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.datemas.de!rt.uk.eu.org!newsfeed.xs4all.nl!newsfeed2a.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 0.016 X-Spam-Evidence: '*H*': 0.97; '*S*': 0.00; 'iterate': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'jan': 0.12; '(k,': 0.16; 'charles': 0.16; 'deleted,': 0.16; 'dictionary.': 0.16; 'expected):': 0.16; 'iteration': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'reedy': 0.16; 'subject: \n ': 0.16; 'wrote:': 0.18; 'separate': 0.22; 'header:User-Agent:1': 0.23; 'stick': 0.24; "i've": 0.25; 'references': 0.26; 'header:X-Complaints-To:1': 0.27; 'header:In- Reply-To:1': 0.27; 'gives': 0.31; 'keys': 0.31; 'way?': 0.31; 'third': 0.33; 'but': 0.35; 'there': 0.35; 'should': 0.36; 'half': 0.37; 'list': 0.37; 'to:addr:python-list': 0.38; 'list,': 0.38; 'pm,': 0.38; 'expect': 0.39; 'delete': 0.39; 'to:addr:python.org': 0.39; 'changed': 0.39; 'received:org': 0.40; 'even': 0.60; 'new': 0.61; 'received:173': 0.61; 'save': 0.62; 'such': 0.63; 'more': 0.64; '*and*': 0.84; 'actually,': 0.84; 'dict,': 0.84; 'received:fios.verizon.net': 0.84 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Terry Reedy Subject: Re: Proper deletion of selected items during map iteration in for loop Date: Fri, 25 Apr 2014 14:50:57 -0400 References: <535AA12A.1030203@earthlink.net> <535AA3B8.3080308@mrabarnett.plus.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: pool-173-75-254-207.phlapa.fios.verizon.net User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 In-Reply-To: <535AA3B8.3080308@mrabarnett.plus.com> 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: 35 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1398451902 news.xs4all.nl 2883 [2001:888:2000:d::a6]:38851 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:70614 On 4/25/2014 2:04 PM, Matthew Barnett wrote: > On 2014-04-25 18:53, Charles Hixson wrote: >> What is the proper way to delete selected items during iteration of a >> map? What I want to do is: >> >> for (k, v) in m.items(): >> if f(k): >> # do some processing of v and save result elsewhere >> del m[k] >> >> But this gives (as should be expected): >> RuntimeError: dictionary changed size during iteration >> In the past I've accumulated the keys to be deleted in a separate list, >> but this time there are likely to be a large number of them, so is there >> some better way? >> > The other way is to build a new dictionary. If you expect to delete more than half the keys *and* if there are no other references to the dict, such that you need the particular object mutated, this might be better. > Actually, there's a third way. Iterate over a snapshot: > > for (k, v) in list(m.items()): > if f(k): > # do some processing of v and save result elsewhere > del m[k] Since a pre-snapshot of items or even just keys will be longer than a list of keys to be deleted, I would stick with the latter. -- Terry Jan Reedy