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Re: NaN comparisons - Call For Anecdotes

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To python-list@python.org
From Terry Reedy <tjreedy@udel.edu>
Subject Re: NaN comparisons - Call For Anecdotes
Date Wed, 09 Jul 2014 00:57:03 -0400
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On 7/8/2014 8:10 PM, Steven D'Aprano wrote:

> There's no force of law that requires Python to enforce reflexivity on
> values where reflexivity does not apply,

There are laws of logic that make the lack of reflexivity obnoxious when 
putting objects in collections. Python evaded the problem, at least for 
some builtins, by contradicting itself and treating nans as equal to 
themselves in the context of collections.

In 2.x, 'in' was defined in terms of ==, but
 >>> nan=float('nan')
 >>> nl = [nan]
 >>> nan in nl
True
even though nan != the only member of nl.

In 3.x, 'in' was redefined to include 'is' as well as '=='.

-- 
Terry Jan Reedy

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Thread

NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-08 16:53 +0200
  Re: NaN comparisons - Call For Anecdotes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-08 17:13 +0000
    Re: NaN comparisons - Call For Anecdotes Chris Angelico <rosuav@gmail.com> - 2014-07-09 03:21 +1000
    Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-08 21:02 +0200
      Re: NaN comparisons - Call For Anecdotes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-09 00:10 +0000
        Re: NaN comparisons - Call For Anecdotes Terry Reedy <tjreedy@udel.edu> - 2014-07-09 00:57 -0400
          Re: NaN comparisons - Call For Anecdotes Steven D'Aprano <steve@pearwood.info> - 2014-07-09 06:43 +0000
            Re: NaN comparisons - Call For Anecdotes Chris Angelico <rosuav@gmail.com> - 2014-07-09 16:52 +1000
        Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-09 17:08 +0200
          Re: NaN comparisons - Call For Anecdotes Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-07-09 16:53 +0000
            Re: NaN comparisons - Call For Anecdotes Ian Kelly <ian.g.kelly@gmail.com> - 2014-07-09 11:26 -0600
            Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-09 19:44 +0200
        Re: NaN comparisons - Call For Anecdotes Chris Angelico <rosuav@gmail.com> - 2014-07-10 01:13 +1000
        Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-09 18:24 +0200
    Re: NaN comparisons - Call For Anecdotes "Anders J. Munch" <2014@jmunch.dk> - 2014-07-08 21:25 +0200
  Re: NaN comparisons - Call For Anecdotes Rustom Mody <rustompmody@gmail.com> - 2014-07-09 20:07 -0700

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