Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.008 X-Spam-Evidence: '*H*': 0.98; '*S*': 0.00; 'builtin': 0.09; 'expectation': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'type,': 0.09; '*always*': 0.16; 'checking,': 0.16; 'choice,': 0.16; 'finney': 0.16; 'garrett': 0.16; 'hasattr(obj,': 0.16; 'ignoring': 0.16; "object's": 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'written.': 0.16; 'exception': 0.16; 'typing': 0.19; 'header:User-Agent:1': 0.23; 'bytes': 0.24; 'header:X-Complaints-To:1': 0.27; 'function': 0.29; 'raise': 0.29; "doesn't": 0.30; 'code': 0.31; 'usually': 0.31; 'provided,': 0.31; 'writes:': 0.31; 'probably': 0.32; 'something': 0.35; 'objects': 0.35; 'but': 0.35; 'there': 0.35; 'returning': 0.36; 'should': 0.36; 'wrong': 0.37; 'being': 0.38; 'ben': 0.38; 'needed': 0.38; 'to:addr:python-list': 0.38; 'use.': 0.39; 'to:addr:python.org': 0.39; 'received:org': 0.40; 'how': 0.40; 'skip:u 10': 0.60; 'times': 0.62; 'reach': 0.63; 'great': 0.65; 'skip:\xe2 10': 0.65; 'believe': 0.68; 'received:125': 0.84; '_free': 0.91; 'choice.': 0.93 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Ben Finney Subject: Re: isiter builtin Date: Sun, 16 Nov 2014 19:23:36 +1100 References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Gmane-NNTP-Posting-Host: jigong.madmonks.org X-Public-Key-ID: 0xAC128405 X-Public-Key-Fingerprint: 517C F14B B2F3 98B0 CB35 4855 B8B2 4C06 AC12 8405 X-Public-Key-URL: http://www.benfinney.id.au/contact/bfinney-pubkey.asc X-Post-From: Ben Finney User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4 (gnu/linux) Cancel-Lock: sha1:K1/aOHKuhNo4eRuIZLXosVrRbHg= 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: 34 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1416126228 news.xs4all.nl 2907 [2001:888:2000:d::a6]:58860 X-Complaints-To: abuse@xs4all.nl Path: csiph.com!usenet.pasdenom.info!bete-des-vosges.org!feed.ac-versailles.fr!nerim.net!novso.com!newsfeed.xs4all.nl!newsfeed4a.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail Xref: csiph.com comp.lang.python:80863 Garrett Berg writes: > However, there are times when I want to do type checking, and the > builtin function *isinstance* is of great use. I would advise that when you think you want type checking, you are probably being overly restrictive. > However, this function fails to be satisfactory in returning whether > the object is a valid iterator. The call hasattr(obj, '__iter__') also > fails because str and bytes types both have that Yes, because both ‘str’ and ‘bytes’ objects are valid iterables. Using ‘isinstance’ in particular is a code smell: it breaks duck typing (checking not for the object's type, but for how the object behaves, which is usually what matters) and is almost always the wrong choice. Not *always* the wrong choice, which is why it's provided, and which is why I call it a code smell: a strong but not infallible sign something is wrong with the code as written. When you reach for ‘isinstance’ (LBYL, which is un-Pythonic), instead ask yourself: ignoring what type this object is, how do I want it to behave, and should I just use it in the expectation that it will raise an exception if it doesn't support that (EAFP, which is very Pythonic)? -- \ “What is needed is not the will to believe but the will to find | `\ out, which is the exact opposite.” —Bertrand Russell, _Free | _o__) Thought and Official Propaganda_, 1928 | Ben Finney