Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #51452

Re: Unexpected results comparing float to Fraction

Path csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!news2.arglkargh.de!news.mixmin.net!rt.uk.eu.org!newsfeed.xs4all.nl!newsfeed3.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Return-Path <python@mrabarnett.plus.com>
X-Original-To python-list@python.org
Delivered-To python-list@mail.python.org
X-Spam-Status OK 0.048
X-Spam-Evidence '*H*': 0.90; '*S*': 0.00; 'float': 0.07; 'python': 0.11; 'coercions': 0.16; 'from:addr:mrabarnett.plus.com': 0.16; 'from:addr:python': 0.16; 'from:name:mrab': 0.16; 'message- id:@mrabarnett.plus.com': 0.16; 'unexpected': 0.16; 'wrote:': 0.18; 'all,': 0.19; 'import': 0.22; 'this?': 0.23; 'header:User- Agent:1': 0.23; 'comparing': 0.24; 'fraction': 0.24; 'header:In- Reply-To:1': 0.27; "i'm": 0.30; 'gives': 0.31; 'apparently': 0.31; "d'aprano": 0.31; 'steven': 0.31; 'convert': 0.35; 'but': 0.35; 'there': 0.35; 'false': 0.36; 'doing': 0.36; 'should': 0.36; 'expected': 0.38; 'to:addr:python-list': 0.38; 'to:addr:python.org': 0.39; 'such': 0.63; 'more': 0.64; 'header :Reply-To:1': 0.67; 'reply-to:no real name:2**0': 0.71; 'float,': 0.84; 'reply-to:addr:python.org': 0.84; 'subject:results': 0.91
X-CM-Score 0.00
X-CNFS-Analysis v=2.1 cv=KrN0hwmN c=1 sm=1 tr=0 a=0nF1XD0wxitMEM03M9B4ZQ==:117 a=0nF1XD0wxitMEM03M9B4ZQ==:17 a=0Bzu9jTXAAAA:8 a=K2DDQYBT4xIA:10 a=POziZqpqTb0A:10 a=ihvODaAuJD4A:10 a=OUOv7kDek9cA:10 a=8nJEP1OIZ-IA:10 a=EBOSESyhAAAA:8 a=8AHkEIZyAAAA:8 a=qLDMBdlwUyoA:10 a=RdPq5cUHhR6Bl2yKv-QA:9 a=wPNLvfGTeEIA:10
X-AUTH mrabarnett:2500
Date Mon, 29 Jul 2013 17:09:24 +0100
From MRAB <python@mrabarnett.plus.com>
User-Agent Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130620 Thunderbird/17.0.7
MIME-Version 1.0
To python-list@python.org
Subject Re: Unexpected results comparing float to Fraction
References <51f68d9c$0$30000$c3e8da3$5496439d@news.astraweb.com>
In-Reply-To <51f68d9c$0$30000$c3e8da3$5496439d@news.astraweb.com>
Content-Type text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding 7bit
X-BeenThere python-list@python.org
X-Mailman-Version 2.1.15
Precedence list
Reply-To python-list@python.org
List-Id General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe <http://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive <http://mail.python.org/pipermail/python-list/>
List-Post <mailto:python-list@python.org>
List-Help <mailto:python-list-request@python.org?subject=help>
List-Subscribe <http://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups comp.lang.python
Message-ID <mailman.5239.1375114343.3114.python-list@python.org> (permalink)
Lines 28
NNTP-Posting-Host 2001:888:2000:d::a6
X-Trace 1375114343 news.xs4all.nl 15904 [2001:888:2000:d::a6]:52770
X-Complaints-To abuse@xs4all.nl
Xref csiph.com comp.lang.python:51452

Show key headers only | View raw

On 29/07/2013 16:43, 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?
>
I'm surprised that Fraction(1/3) != Fraction(1, 3); after all, floats
are approximate anyway, and the float value 1/3 is more likely to be
Fraction(1, 3) than Fraction(6004799503160661, 18014398509481984).

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 15:43 +0000
  Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 09:50 -0600
  Re: Unexpected results comparing float to Fraction MRAB <python@mrabarnett.plus.com> - 2013-07-29 17:09 +0100
  Re: Unexpected results comparing float to Fraction Chris Angelico <rosuav@gmail.com> - 2013-07-29 17:20 +0100
    Re: Unexpected results comparing float to Fraction Rotwang <sg552@hotmail.co.uk> - 2013-07-29 19:20 +0100
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 19:32 +0000
  Re: Unexpected results comparing float to Fraction MRAB <python@mrabarnett.plus.com> - 2013-07-29 17:48 +0100
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 17:27 +0000
  Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 10:40 -0600
    Re: Unexpected results comparing float to Fraction Rotwang <sg552@hotmail.co.uk> - 2013-07-29 19:16 +0100
      Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 13:33 -0600
  Re: Unexpected results comparing float to Fraction MRAB <python@mrabarnett.plus.com> - 2013-07-29 18:04 +0100
    Re: Unexpected results comparing float to Fraction Grant Edwards <invalid@invalid.invalid> - 2013-07-29 18:46 +0000
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 20:19 +0000
  Re: Unexpected results comparing float to Fraction Terry Reedy <tjreedy@udel.edu> - 2013-07-29 13:08 -0400
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-29 17:29 +0000
      Re: Unexpected results comparing float to Fraction Terry Reedy <tjreedy@udel.edu> - 2013-07-29 16:48 -0400
  Re: Unexpected results comparing float to Fraction Chris Angelico <rosuav@gmail.com> - 2013-07-29 18:14 +0100
  Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 11:35 -0600
  Re: Unexpected results comparing float to Fraction Serhiy Storchaka <storchaka@gmail.com> - 2013-07-29 22:34 +0300
  Re: Unexpected results comparing float to Fraction Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-29 14:35 -0600
  Unexpected results comparing float to Fraction Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-07-30 14:32 +0100
  Re: Unexpected results comparing float to Fraction Tony the Tiger <tony@tiger.invalid> - 2013-07-31 15:23 -0500
    Re: Unexpected results comparing float to Fraction Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-08-01 06:20 +0000
      Re: Unexpected results comparing float to Fraction Chris Angelico <rosuav@gmail.com> - 2013-08-01 07:32 +0100
      Re: Unexpected results comparing float to Fraction Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-08-01 10:44 +0100
      Re: Unexpected results comparing float to Fraction Chris Angelico <rosuav@gmail.com> - 2013-08-01 10:48 +0100

csiph-web