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Groups > comp.lang.python > #73603

Re: python 3.44 float addition bug?

From Ben Finney <ben@benfinney.id.au>
Subject Re: python 3.44 float addition bug?
Date 2014-06-26 13:39 +1000
References <XnsA35313E634BA0fraserlonggmailcom34@216.196.109.145> <645be4a7-2e1b-44a4-9c45-9184c6df5518@googlegroups.com> <53ab8bc4$0$11121$c3e8da3@news.astraweb.com>
Newsgroups comp.lang.python
Message-ID <mailman.11251.1403753977.18130.python-list@python.org> (permalink)

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Steven D'Aprano <steve@pearwood.info> writes:

> On Wed, 25 Jun 2014 14:12:31 -0700, Maciej Dziardziel wrote:
>
> > Floating points values use finite amount of memory, and cannot
> > accurately represent infinite amount of numbers, they are only
> > approximations. This is limitation of float type and applies to any
> > languages that uses types supported directly by cpu. To deal with it
> > you can either use decimal.Decimal type that operates using decimal
> > system and saves you from such surprises
>
> That's a myth. decimal.Decimal *is* a floating point value

That's misleading: Decimal uses *a* floating-point representation, but
not the one commonly referred to. That is, Decimal does not use IEEE-754
floating point.

> and is subject to *exactly* the same surprises as binary floats,

Since those “surprises” are the ones inherent to *decimal*, not binary,
floating point, I'd say it's also misleading to refer to them as
“exactly the same surprises”. They're barely surprises at all, to
someone raised on decimal notation.

This makes the Decimal functionality starkly different from the built-in
‘float’ type, and it *does* save you from the rather-more-surprising
behaviour of the ‘float’ type. This is not mythical.

-- 
 \     “Guaranteed to work throughout its useful life.” —packaging for |
  `\                                          clockwork toy, Hong Kong |
_o__)                                                                  |
Ben Finney

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Thread

python 3.44 float addition bug? FraserL <fraser.long+usenet@NOSPAMgmail.com> - 2014-06-20 19:57 -0500
  Re: python 3.44 float addition bug? FraserL <fraser.long+usenet@gmail.com> - 2014-06-20 20:11 -0500
    Re: python 3.44 float addition bug? Gary Herron <gary.herron@islandtraining.com> - 2014-06-20 18:19 -0700
  Re: python 3.44 float addition bug? Gary Herron <gary.herron@islandtraining.com> - 2014-06-20 18:07 -0700
  Re: python 3.44 float addition bug? Chris Angelico <rosuav@gmail.com> - 2014-06-21 11:33 +1000
  Re: python 3.44 float addition bug? INADA Naoki <songofacandy@gmail.com> - 2014-06-21 10:06 +0900
  Re: python 3.44 float addition bug? Grant Edwards <invalid@invalid.invalid> - 2014-06-21 14:25 +0000
  Re: python 3.44 float addition bug? Ned Deily <nad@acm.org> - 2014-06-21 12:24 -0700
    Re: python 3.44 float addition bug? buck <workitharder@gmail.com> - 2014-06-23 17:55 -0700
      Re: python 3.44 float addition bug? Chris Angelico <rosuav@gmail.com> - 2014-06-24 13:26 +1000
      Re: python 3.44 float addition bug? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-06-24 17:30 +1200
      Re: python 3.44 float addition bug? Steven D'Aprano <steve@pearwood.info> - 2014-06-24 06:34 +0000
  Re: python 3.44 float addition bug? Maciej Dziardziel <fiedzia@gmail.com> - 2014-06-25 14:12 -0700
    Re: python 3.44 float addition bug? Steven D'Aprano <steve@pearwood.info> - 2014-06-26 02:56 +0000
      Re: python 3.44 float addition bug? Chris Angelico <rosuav@gmail.com> - 2014-06-26 13:13 +1000
        Re: python 3.44 float addition bug? Steven D'Aprano <steve@pearwood.info> - 2014-06-26 04:17 +0000
          Re: python 3.44 float addition bug? Chris Angelico <rosuav@gmail.com> - 2014-06-26 14:41 +1000
      Re: python 3.44 float addition bug? Ben Finney <ben@benfinney.id.au> - 2014-06-26 13:39 +1000
        Re: python 3.44 float addition bug? Steven D'Aprano <steve@pearwood.info> - 2014-06-26 09:15 +0000
          Re: python 3.44 float addition bug? Chris Angelico <rosuav@gmail.com> - 2014-06-26 19:38 +1000
            Re: python 3.44 float addition bug? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-06-27 02:51 +0000
              Re: python 3.44 float addition bug? Chris Angelico <rosuav@gmail.com> - 2014-06-27 13:24 +1000
              Re: python 3.44 float addition bug? Ian Kelly <ian.g.kelly@gmail.com> - 2014-06-27 10:18 -0600
      Re: python 3.44 float addition bug? Stefan Behnel <stefan_ml@behnel.de> - 2014-06-26 07:53 +0200

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