Path: csiph.com!usenet.pasdenom.info!news.redatomik.org!newsfeed.xs4all.nl!newsfeed8.news.xs4all.nl!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 0.007 X-Spam-Evidence: '*H*': 0.99; '*S*': 0.00; 'true,': 0.04; 'false,': 0.07; 'subject:skip:c 10': 0.07; 'cc:addr:python-list': 0.09; 'behave': 0.09; 'incorrect': 0.09; 'none"': 0.09; 'pavel': 0.09; 'python': 0.10; '2.7': 0.13; 'wed,': 0.15; 'result.': 0.15; '2.6:': 0.16; 'clauses': 0.16; 'false),': 0.16; 'from:addr:rosuav': 0.16; 'from:name:chris angelico': 0.16; 'hint': 0.16; 'typo': 0.16; 'wrote:': 0.16; 'language': 0.19; '2015': 0.20; 'cc:2**0': 0.20; 'cc:addr:python.org': 0.20; 'aug': 0.20; 'visible': 0.22; 'code,': 0.23; 'header:In-Reply-To:1': 0.24; 'feature': 0.24; 'wondering': 0.25; 'subject:list': 0.26; 'least': 0.27; 'message-id:@mail.gmail.com': 0.27; 'values': 0.28; 'accepts': 0.29; 'branch': 0.30; 'certainly': 0.30; "can't": 0.32; 'possibly': 0.32; 'though,': 0.32; '(for': 0.34; 'that,': 0.34; 'list': 0.34; 'received:google.com': 0.35; 'something': 0.35; 'but': 0.36; 'possible': 0.36; "wasn't": 0.36; 'pm,': 0.36; 'subject:: ': 0.37; 'release': 0.37; 'doing': 0.38; 'version': 0.38; 'stuff': 0.38; 'hi,': 0.38; 'why': 0.39; 'test': 0.39; 'subject:with': 0.40; 'some': 0.40; 'your': 0.60; 'skip:u 10': 0.61; 'back': 0.62; 'improved': 0.63; 'is.': 0.63; 'grab': 0.64; 'was:': 0.66; 'results': 0.66; 'finally': 0.70; 'yourself': 0.73; "'for'": 0.84; '2.7.': 0.84; 'chrisa': 0.84; 'expect.': 0.84; 'to:none': 0.91; 'look.': 0.91 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:cc :content-type; bh=Q/Yp8okHzUvmuTen3JQup5fxKGPSQi+FYrQQbdDlrvA=; b=PHvbmbTlT2i5cgrvaOSX7ShfCZ1v1y2WPv+FCPfI8m6jPNuJbDBbVOm1yMbyFjq9Oq a6UtpJTHTCE8Wl1SLqB1jmefhS4azUQlvUW1FbOI1yuOH2zNfq2X0pxH+73pukkSAvn2 BnX0/lJ0b0swRlv50jjkqZkk1yY4LPpgGcwgAFRYrn+sXqb8nZXWzVu8XzYUXO8VX9m6 t7j9n5jtkCnrsq3ouZZH/OlF1HN0TPoDVTu0P7YPoMCHrGA28I099KQLDuM0XYTKj388 h/CNu69EMB5C9hHUKa/Dy1sm2bYFyf7IGs8IZgVucL2ZJMEC27xCWNHs+BhNtnkVKygm sOBg== MIME-Version: 1.0 X-Received: by 10.50.109.233 with SMTP id hv9mr4291873igb.92.1438758350020; Wed, 05 Aug 2015 00:05:50 -0700 (PDT) In-Reply-To: References: Date: Wed, 5 Aug 2015 17:05:49 +1000 Subject: Re: GOTCHA with list comprehension From: Chris Angelico Cc: "python-list@python.org" Content-Type: text/plain; charset=UTF-8 X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.20+ 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: 1438758358 news.xs4all.nl 2826 [2001:888:2000:d::a6]:55161 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:94999 On Wed, Aug 5, 2015 at 4:48 PM, Pavel S wrote: > Hi, > > I recently found interesting GOTCHA while doing list comprehension in python 2.6: > >>>> values = ( True, False, 1, 2, 3, None ) >>>> [ value for value in values if value if not None ] > [True, 1, 2, 3] > > I was wondering why this list comprehension returns incorrect results and finally found a typo in the condition. The typo wasn't visible at the first look. > > My intention was: if value is not None > But I wrote: if value if not None > > Is that a language feature of list comprehension that it accepts conditions like: if A if B if C if D ...? It certainly is. You can chain 'for' and 'if' clauses as much as you like, and they behave exactly the way you'd expect. You might possibly get a warning from a linter with your code, though, as it has an always-true condition ("if not None" can never be false), so it's possible something might hint at what's going on; but other than that, all you can do is test stuff and see if it's giving the right result. Incidentally, why Python 2.6? Python 2.7 has been out for a pretty long time now, and if you can't move to version 3.x, I would at least recommend using 2.7. Since the release of 2.6.9 back before Frozen came out, that branch has been completely unmaintained. Grab yourself a 2.7 and take advantage of some neat new features (for old values of "new"), and improved compatibility with 3.x. ChrisA