Path: csiph.com!usenet.pasdenom.info!dedibox.gegeweb.org!gegeweb.eu!nntpfeed.proxad.net!proxad.net!feeder1-2.proxad.net!news.tele.dk!news.tele.dk!small.news.tele.dk!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; 'cursor': 0.09; 'derived': 0.09; 'inserts': 0.09; 'item,': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'subject:while': 0.09; 'python': 0.11; '(there': 0.16; '>they': 0.16; 'confuse': 0.16; 'expects': 0.16; 'message-id:@4ax.com': 0.16; 'programmer,': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'either.': 0.24; 'url:home': 0.24; 'possibly': 0.26; 'query': 0.26; 'this:': 0.26; 'skip:" 20': 0.27; 'header:X-Complaints- To:1': 0.27; "doesn't": 0.30; 'matching': 0.30; 'code': 0.31; 'that.': 0.31; "d'aprano": 0.31; 'steven': 0.31; 'says': 0.33; 'could': 0.34; 'something': 0.35; 'there': 0.35; 'really': 0.36; 'object,': 0.36; 'method': 0.36; 'charset:us-ascii': 0.36; 'should': 0.36; 'experience,': 0.37; 'received:76': 0.38; 'nov': 0.38; 'to:addr:python-list': 0.38; 'list,': 0.38; 'fact': 0.38; 'rather': 0.38; 'does': 0.39; 'to:addr:python.org': 0.39; 'received:org': 0.40; 'how': 0.40; 'remove': 0.60; 'middle': 0.60; 'removing': 0.60; 'most': 0.60; 'show': 0.63; 'subject:For': 0.78; 'items,': 0.91; '2013': 0.98 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Dennis Lee Bieber Subject: Re: For-each behavior while modifying a collection Date: Fri, 29 Nov 2013 11:37:45 -0500 Organization: IISS Elusive Unicorn References: <52980885$0$29993$c3e8da3$5496439d@news.astraweb.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: adsl-76-249-16-78.dsl.klmzmi.sbcglobal.net X-Newsreader: Forte Agent 6.00/32.1186 X-No-Archive: YES 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: 40 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1385743058 news.xs4all.nl 15908 [2001:888:2000:d::a6]:60924 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:60771 On 29 Nov 2013 03:22:45 GMT, Steven D'Aprano declaimed the following: >Even if there was a counter, how should it be modified? The code you show >was this: > >def keepByValue(self, key=None, value=[]): > for row in self.flows: > if not row[key] in value: > self.flows.remove(row) > > >What exactly does the remove() method do? How do you know? > >self.flows could be *any object at all*, it won't be known until run- >time. The remove method could do *anything*, that won't be known until >runtime either. Just because you, the programmer, expects that self.flows >will be a list, and that remove() will remove at most one item, doesn't >mean that Python can possibly know that. Perhaps self.flows returns an >subclass of list, and remove() will remove all of the matching items, not >just one. Perhaps it is some other object, and rather than removing >anything, in fact it actually inserts extra items in the middle of the >sequence. (There is no law that says that methods must do what they say >they do.) > Let's really confuse matters... Say "self.flows" is something derived from a database cursor/result set... Then "self.flows.remove(row)" could: 1) remove the row from the cursor/result set [iterating over a cursor tends, in my experience, to use up the items]; 2) execute a query to remove the matching row from the database itself; 3) do both... -- Wulfraed Dennis Lee Bieber AF6VN wlfraed@ix.netcom.com HTTP://wlfraed.home.netcom.com/