Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!news.mixmin.net!rt.uk.eu.org!newsfeed.xs4all.nl!newsfeed4a.news.xs4all.nl!xs4all!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 0.021 X-Spam-Evidence: '*H*': 0.96; '*S*': 0.00; 'exists.': 0.07; 'clean.': 0.09; 'will,': 0.09; 'cc:addr:python-list': 0.11; 'def': 0.12; "(it's": 0.16; 'buggy': 0.16; 'from:addr:rosuav': 0.16; 'from:name:chris angelico': 0.16; 'length.': 0.16; 'subject:dictionaries': 0.16; "value',": 0.16; "{'a':": 0.16; 'wrote:': 0.18; 'cheap': 0.19; 'seems': 0.21; 'appears': 0.22; 'cc:addr:python.org': 0.22; 'cc:2**0': 0.24; 'define': 0.26; 'header:In-Reply-To:1': 0.27; 'words': 0.29; 'message- id:@mail.gmail.com': 0.30; 'getting': 0.31; "skip:' 10": 0.31; '>>>>': 0.31; 'subject:with': 0.35; 'but': 0.35; 'received:google.com': 0.35; 'there': 0.35; 'list': 0.37; 'skip:[ 10': 0.38; 'pm,': 0.38; 'little': 0.38; 'even': 0.60; 'skip:o 30': 0.61; 'situation': 0.65; 'side': 0.67; 'frank': 0.68; 'skip:w 30': 0.69; 'default': 0.69; 'abc': 0.84; 'effects,': 0.84; 'to:none': 0.92 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=Goa4COW7tmqx/a4mqFXIz7NtzNnINNw9dDSbCRfzr8o=; b=b/C7WKcADRWZu6nRjWlwxIDtvrP5MaKDQT0c9pKd5CA4GH9b6SUznOVeFoB1/HlKNT uG9XdEWBfVPdmO6jJ+hCrpbpJHS1qc22r+wByB76fTJRM+jlkhIDagdxycaGqVvBRJb1 vtHApbmS+VjsSidW/OYQEt49Uz4abU4NLc1ZFguaglo6PA3rwBnVZInIHzoHJpuK1DyH zjTM/IzniHJAi15FVcxH8CZ42QqZPX1qHXupOjxcavEl4OBr6rxdZOkesfWzyXqM7vHL /5IWWTJAVjan1slVIKMoXv1r9X8P+JQUzyBSztCawF4CrtaMMtkVWD5zXdv4l+M0Dvzl R0yg== MIME-Version: 1.0 X-Received: by 10.69.31.74 with SMTP id kk10mr2601393pbd.169.1396944028601; Tue, 08 Apr 2014 01:00:28 -0700 (PDT) In-Reply-To: References: <534105ce$0$1365$4fafbaef@reader1.news.tin.it> <21ef5159-ad95-4d43-a2d6-7ecda941d978@googlegroups.com> Date: Tue, 8 Apr 2014 18:00:28 +1000 Subject: Re: Keeping track of things with dictionaries From: Chris Angelico Cc: "python-list@python.org" Content-Type: text/plain; charset=UTF-8 X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.15 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: 33 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1396944031 news.xs4all.nl 2884 [2001:888:2000:d::a6]:52556 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:69842 On Tue, Apr 8, 2014 at 5:14 PM, Frank Millman wrote: > It appears that when you use 'setdefault', the default is always evaluated, > even if the key exists. > >>>> def get_value(val): > ... print('getting value', val) > ... return val*2 > ... >>>> my_dict = {} >>>> my_dict.setdefault('a', get_value('xyz')) > getting value xyz > 'xyzxyz' >>>> my_dict.setdefault('a', get_value('abc')) > getting value abc > 'xyzxyz' >>>> my_dict > {'a': 'xyzxyz'} >>>> > > It seems odd. Is there a situation where this behaviour is useful? If the default value is cheap to define and has no side effects, it can be very clean. words_by_length = {} for word in open("/usr/share/dict/words"): words_by_length.setdefault(len(word), []).append(word) This will, very conveniently, give you a list of all words of a particular length. (It's actually a little buggy but you get the idea.) ChrisA