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Re: Unexpected results comparing float to Fraction

From Terry Reedy <tjreedy@udel.edu>
Subject Re: Unexpected results comparing float to Fraction
Date 2013-07-29 16:48 -0400
References <51f68d9c$0$30000$c3e8da3$5496439d@news.astraweb.com> <CALwzidmd_e54uEiek7OEH95ezARWEB89hCWfzFMtmi+jdU9o7A@mail.gmail.com> <mailman.5246.1375117736.3114.python-list@python.org> <51f6a68c$0$30000$c3e8da3$5496439d@news.astraweb.com>
Newsgroups comp.lang.python
Message-ID <mailman.5268.1375130932.3114.python-list@python.org> (permalink)

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On 7/29/2013 1:29 PM, Steven D'Aprano wrote:
> On Mon, 29 Jul 2013 13:08:20 -0400, Terry Reedy wrote:
>
>> 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
>
> This is Python, and we can make __eq__ methods that do anything,
> including be non-transitive, non-reflexive, and nonsensical if we like :-)

Yes, Python's developers can intentionally introduce bugs, but we try 
not to. The definitions of sets and dicts and containment assume that == 
means equality as mathematically defined. As one time, we had 0 == 0.0 
and 0 == Decimal(0) but 0.0 != Decimal(0) (and so on for all integral 
float values. That 'misfeature' was corrected because of the 'problems' 
it caused. That lesson learned, one of the design requirements for the 
new enum class (metaclass) was that it not re-introduce non-transitivity.

-- 
Terry Jan Reedy

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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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