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Re: Unexpected results comparing float to Fraction

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From Ian Kelly <ian.g.kelly@gmail.com>
Date Mon, 29 Jul 2013 09:50:34 -0600
Subject Re: Unexpected results comparing float to Fraction
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On Mon, Jul 29, 2013 at 9:43 AM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> 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.  So from that standpoint it makes sense to me to cast
to Fraction when comparing.

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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

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