Path: csiph.com!newsfeed.hal-mli.net!feeder3.hal-mli.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!newsfeed.xs4all.nl!newsfeed4.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 0.018 X-Spam-Evidence: '*H*': 0.96; '*S*': 0.00; 'float': 0.07; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'python': 0.11; 'jan': 0.12; 'coercions': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'reedy': 0.16; 'unequal': 0.16; 'unexpected': 0.16; 'wrote:': 0.18; 'import': 0.22; 'otherwise,': 0.22; 'this?': 0.23; 'header:User-Agent:1': 0.23; 'comparing': 0.24; 'fraction': 0.24; 'instance,': 0.24; 'mon,': 0.24; 'header:X -Complaints-To:1': 0.27; 'header:In-Reply-To:1': 0.27; 'am,': 0.29; 'gives': 0.31; 'apparently': 0.31; "d'aprano": 0.31; 'steven': 0.31; 'becomes': 0.33; 'sense': 0.34; 'convert': 0.35; 'but': 0.35; 'there': 0.35; 'false': 0.36; 'words,': 0.36; 'doing': 0.36; 'should': 0.36; 'expected': 0.38; 'to:addr:python- list': 0.38; 'to:addr:python.org': 0.39; 'received:org': 0.40; 'ian': 0.60; 'received:173': 0.61; 'such': 0.63; 'skip:n 10': 0.64; 'jul': 0.74; 'float,': 0.84; 'received:fios.verizon.net': 0.84; 'cast': 0.91; 'subject:results': 0.91; '2013': 0.98 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Terry Reedy Subject: Re: Unexpected results comparing float to Fraction Date: Mon, 29 Jul 2013 13:08:20 -0400 References: <51f68d9c$0$30000$c3e8da3$5496439d@news.astraweb.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: pool-173-75-251-66.phlapa.fios.verizon.net User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20130620 Thunderbird/17.0.7 In-Reply-To: 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: 42 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1375117736 news.xs4all.nl 16001 [2001:888:2000:d::a6]:34667 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:51463 On 7/29/2013 11:50 AM, Ian Kelly wrote: > On Mon, Jul 29, 2013 at 9:43 AM, Steven D'Aprano > wrote: >> Comparing floats to Fractions gives unexpected results: >> >> # Python 3.3 >> py> from fractions import Fraction >> py> 1/3 == Fraction(1, 3) >> False >> >> but: >> >> py> 1/3 == float(Fraction(1, 3)) >> True >> >> >> I expected that float-to-Fraction comparisons would convert the Fraction >> to a float, but apparently they do the opposite: they convert the float >> to a Fraction: >> >> py> Fraction(1/3) >> Fraction(6004799503160661, 18014398509481984) >> >> >> Am I the only one who is surprised by this? Is there a general rule for >> which way numeric coercions should go when doing such comparisons? > > Any float can be precisely represented as a Fraction. Not so in the > other direction. In other words, there can be multiple unequal Franctions that have the same float value: for instance, Fraction(1,3) and Fraction(6004799503160661, 18014398509481984) > So from that standpoint it makes sense to me to cast to > Fraction when comparing. Otherwise, == becomes non-transitive -- Terry Jan Reedy