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Re: Is nan in (nan,) correct?

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From Ben Finney <ben+python@benfinney.id.au>
Subject Re: Is nan in (nan,) correct?
Date Fri, 06 Mar 2015 14:26:15 +1100
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Ethan Furman <ethan@stoneleaf.us> writes:

> On 03/05/2015 06:55 PM, Ben Finney wrote:
>
> >     class NullType(object):
> >         """ A type whose value never equals any other.
> > 
> >             This type's values will behave correctly when tested for
> >             membership in a collection::
> > 
> >                 >>> foo = NullType()
> >                 >>> bar = NullType()
> >                 >>> foo is foo
> >                 True
> >                 >>> foo is bar
> >                 False
> >                 >>> foo == foo
> >                 False
> >                 >>> foo == bar
> >                 False
> >                 >>> quux = [foo, "spam"]
> >                 >>> "spam" in quux
> >                 True
> >                 >>> foo in quux
> >                 True
>
> Did you mean False here?  Because True is current behavior.

Isn't the point at issue that the Python interpreter *may* optimise by
assuming ‘is implies equality’, so the ‘in’ operator can fail if that
assumption is false?

I thought the problem was that types with custom behaviour, as with the
‘NullType’ example, needed to deal specially with the ‘is implies
equality’ optimisation Steven explained.

If that's the correct behaviour, and we can *depend* on it being
correct, then I don't see what the problem is.

-- 
 \          “What I have to do is see, at any rate, that I do not lend |
  `\      myself to the wrong which I condemn.” —Henry Thoreau, _Civil |
_o__)                                                    Disobedience_ |
Ben Finney

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Thread

Is nan in (nan,) correct? random832@fastmail.us - 2015-03-05 17:26 -0500
  Re: Is nan in (nan,) correct? sohcahtoa82@gmail.com - 2015-03-05 15:11 -0800
    Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 10:20 +1100
      Re: Is nan in (nan,) correct? sohcahtoa82@gmail.com - 2015-03-05 15:27 -0800
        Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 10:39 +1100
        Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 10:40 +1100
    Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 10:25 +1100
  Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-06 13:09 +1100
    Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 13:55 +1100
    Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-05 19:18 -0800
    Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 14:26 +1100
    Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-05 19:44 -0800
    Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 14:49 +1100
    Re: Is nan in (nan,) correct? random832@fastmail.us - 2015-03-05 23:37 -0500
      Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 04:07 +1100
    Re: Is nan in (nan,) correct? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-03-06 04:46 +0000
  Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 01:50 -0800
    Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 21:01 +1100
      Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 02:22 -0800
        Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 03:59 +1100
          Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 10:04 -0800
            Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-06 10:16 -0800
      Re: Is nan in (nan,) correct? Grant Edwards <invalid@invalid.invalid> - 2015-03-06 15:34 +0000
    Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 03:43 +1100
      Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 09:04 -0800
        Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-07 04:16 +1100
          Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 09:36 -0800
        Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 10:33 +1100
          Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 18:37 -0800

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