Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!news.albasani.net!newsfeed.freenet.ag!news2.euro.net!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 0.000 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'else:': 0.03; 'clause': 0.07; 'executed': 0.07; 'false.': 0.07; 'skip:/ 10': 0.07; 'python': 0.09; '(although': 0.09; 'from:addr:ethan': 0.09; 'from:addr:stoneleaf.us': 0.09; 'from:name:ethan furman': 0.09; 'loop.': 0.09; 'message-id:@stoneleaf.us': 0.09; 'received:184.172': 0.09; 'received:gator410.hostgator.com': 0.09; 'terry': 0.09; 'useless': 0.09; '~ethan~': 0.09; 'constructs.': 0.16; 'received:69.93': 0.16; 'reedy': 0.16; 'sink': 0.16; 'wrote:': 0.17; 'example': 0.23; "python's": 0.23; 'statement': 0.23; 'header:In-Reply-To:1': 0.25; 'header:User-Agent:1': 0.26; 'am,': 0.27; 'label': 0.27; 'trouble': 0.28; 'statements': 0.29; 'starts': 0.29; 'probably': 0.29; 'code': 0.31; 'point': 0.31; 'to:addr:python-list': 0.33; 'but': 0.36; "i'll": 0.36; 'why': 0.37; 'quite': 0.37; 'subject:: ': 0.38; 'several': 0.39; 'to:addr:python.org': 0.39; 'header:Received:5': 0.40; 'skip:u 10': 0.60; 'real': 0.61; '(that': 0.62; 'subject:...': 0.63; 'times': 0.63; 'often,': 0.84 Date: Tue, 22 Jan 2013 12:09:25 -0800 From: Ethan Furman User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130106 Thunderbird/17.0.2 MIME-Version: 1.0 To: python-list@python.org Subject: Re: Understanding while...else... References: In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - gator410.hostgator.com X-AntiAbuse: Original Domain - python.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - stoneleaf.us X-BWhitelist: yes X-Source: X-Source-Args: X-Source-Dir: X-Source-Sender: ([173.12.184.235]) [173.12.184.235]:46840 X-Source-Auth: ethan+stoneleaf.us X-Email-Count: 1 X-Source-Cap: dG9idWs7dG9idWs7Z2F0b3I0MTAuaG9zdGdhdG9yLmNvbQ== 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: 1358885817 news.xs4all.nl 6897 [2001:888:2000:d::a6]:46087 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:37344 On 01/22/2013 09:44 AM, Terry Reedy wrote: > Several people have trouble understanding Python's while-else and > for-else constructs. It is actually quite simple if one starts with > if-else, which few have any trouble with. > > Start with, for example > > if n > 0: > n -= 1 > else: > n = None > > The else clause is executed if and when the condition is false. (That > the code is useless is not the point here.) Now use pseudo-Python label > and goto statements to repeatedly decrement n > > label: check > if n > 0: > n -= 1 > goto: check > else: > n = None > > The else clause is executed if and when the condition is false. > Now use a real Python while statement to do the *same > thing*. > > while n > 0: > n -= 1 > else: > n = None I understand how it works (although it did take a while for it to sink in); my gripe, and probably why it is misunderstood so often, is that nine times out of ten when I /want/ to use a while-else or for-else I only want the true/false check /once/, at the beginning of the loop. /Occasionally/ I'll actually have use for the search pattern, and then I can use the while- or for-else construct, but that's pretty rare for me. ~Ethan~