Path: csiph.com!newsfeed.hal-mli.net!feeder3.hal-mli.net!newsfeed.hal-mli.net!feeder2.hal-mli.net!newsfeed.xs4all.nl!newsfeed3.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 0.004 X-Spam-Evidence: '*H*': 0.99; '*S*': 0.00; 'handler': 0.05; 'callback': 0.09; 'exception,': 0.09; 'exit': 0.09; 'http': 0.09; 'python:': 0.09; 'seemed': 0.09; 'worked.': 0.09; 'subject:question': 0.10; 'brilliant': 0.16; 'concern,': 0.16; 'dark.': 0.16; 'exceptions,': 0.16; 'from:addr:rosuav': 0.16; 'from:name:chris angelico': 0.16; 'message;': 0.16; 'scare': 0.16; 'subject:exception': 0.16; 'subject:handling': 0.16; 'unexpected': 0.16; 'exception': 0.16; 'all.': 0.16; 'language': 0.16; 'fix': 0.17; 'wrote:': 0.18; 'users.': 0.18; 'wed,': 0.18; 'trying': 0.19; 'properly': 0.19; 'server,': 0.19; 'fit': 0.20; 'thanks.': 0.20; 'error': 0.23; 'browsers': 0.24; 'instance,': 0.24; 'server.': 0.24; 'logging': 0.26; 'somewhere': 0.26; 'header:In- Reply-To:1': 0.27; 'function': 0.29; 'am,': 0.29; 'reporting': 0.29; 'mode': 0.30; 'message-id:@mail.gmail.com': 0.30; "i'm": 0.30; '(which': 0.31; 'code': 0.31; 'block,': 0.31; "d'aprano": 0.31; 'exceptions': 0.31; 'steven': 0.31; 'file': 0.32; 'probably': 0.32; 'running': 0.33; 'received:209.85': 0.35; 'except': 0.35; 'received:209.85.220': 0.35; 'case,': 0.35; 'but': 0.35; 'received:google.com': 0.35; 'there': 0.35; 'acceptable': 0.36; 'crazy': 0.36; 'should': 0.36; 'behind': 0.37; 'received:209': 0.37; 'sometimes': 0.38; 'to:addr:python-list': 0.38; 'rather': 0.38; 'to:addr:python.org': 0.39; 'days': 0.60; 'catch': 0.60; 'problems.': 0.60; 'tell': 0.60; 'full': 0.61; 'matter': 0.61; 'simply': 0.61; 'simple': 0.61; 'skip:n 10': 0.64; 'our': 0.64; 'taking': 0.65; 'details': 0.65; 'saw': 0.77; 'bare': 0.84; 'everything.': 0.84; 'leak': 0.84; 'short,': 0.84; 'story,': 0.91; '2013': 0.98 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:x-received:in-reply-to:references:date:message-id :subject:from:to:content-type; bh=Etu1nCJ/O1f445MrRT/rn+PxWx9lhOyGuoxlGoNjtfo=; b=MZ65pjqgT4Qm0RPVPFbINpnYlJMESN2f3X4tA1j5pAXuabHu7JY9Z0+BOBisTaa8rL b9gDx4daOqpOzpSPH4nQWWX4Wu29RI0P2J/tTgkOAeyTDQZcX/bQV9mudL12jYtMEmCI Y7sSZ5uVf99FvJuMdmceMvhTP8bUG21HPaCCfLzeW32PvJvpCrUsVhw74D52ER/UFP38 TMsazk3/OrtusKY6DKqdeG4xTU5rmZ6FfMV3oB465IrWF5fkPxgxST1PTSFnYFbQrTcH U/eWCLFKZw+DstcIe2erAKREMvbkyjXhUK2V9Kv/gWhMMucjG5uS2KE4Rqahw6Ea6eEG wY8w== MIME-Version: 1.0 X-Received: by 10.220.109.145 with SMTP id j17mr19687939vcp.34.1365524607789; Tue, 09 Apr 2013 09:23:27 -0700 (PDT) In-Reply-To: <51642e32$0$30003$c3e8da3$5496439d@news.astraweb.com> References: <1ae3261b-078d-4362-abff-ea4471addd6a@googlegroups.com> <51642e32$0$30003$c3e8da3$5496439d@news.astraweb.com> Date: Wed, 10 Apr 2013 02:23:27 +1000 Subject: Re: newbie question about confusing exception handling in urllib From: Chris Angelico To: python-list@python.org Content-Type: text/plain; charset=ISO-8859-1 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: 32 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1365524611 news.xs4all.nl 2612 [2001:888:2000:d::a6]:52805 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:43194 On Wed, Apr 10, 2013 at 1:05 AM, Steven D'Aprano wrote: > One exception to this rule (no pun intended) is that sometimes you want > to hide the details of unexpected tracebacks from your users. In that > case, it may be acceptable to wrap your application's main function in a > try block, catch any unexpected exceptions, log the exception, and then > quietly exit with a short, non-threatening error message that won't scare > the civilians This is important to some types of security concern, too; for instance, if I'm running a web server, I probably don't want to leak details of exceptions and tracebacks to a potential attacker. Same again: catch the exception, log it, return simple error message; additionally, you can return that message as an HTTP response rather than simply bombing the web server. But again, a bare except should almost always be logging its exceptions. True story, though not in Python: After taking over the code of an ex-coworker, I was trying to fix some crazy problems. Everything I did seemed to kinda-work, but nothing properly worked. Trying to clean up the code to comply with "use strict" mode (which will tell you what language this is, and it isn't Perl) was a matter of blundering about in the dark. Turned out there was an event handler somewhere that buried the *entire file full of code* behind a callback that caught and suppressed everything. Gee, thanks. Web browsers these days are pretty good at reporting exceptions - we were mainly using Chrome's inbuilt Firebug-equivalent - but our brilliant coworker saw fit to hide them all. Exceptions are a huge boon. ChrisA