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Groups > comp.lang.python > #6543

Re: float("nan") in set or as key

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On 2011-05-29, Wolfgang Rohdewald <wolfgang@rohdewald.de> wrote:
> On Sonntag 29 Mai 2011, Tim Delaney wrote:
>> There's a second part the mystery - sets and dictionaries (and
>> I think lists) assume that identify implies equality (hence
>> the second result). This was recently discussed on
>> python-dev, and the decision was to leave things as-is.
>
> On Sonntag 29 Mai 2011, Grant Edwards wrote:
>> Even if they are the same nan, it's still not equal to itself.
>
> if I understand this thread correctly, they are not equal to itself
> as specified by IEEE

And Python follows that convention.

> but Python treats them equal in sets and dictionaries for performance
> reasons

It treats them as identical (not sure if that's the right word).  The
implementation is checking for ( A is B or A == B ).  Presumably, the
assumpting being that all objects are equal to themselves.  That
assumption is not true for NaN objects, so the buggy behavior is
observed.

-- 
Grant

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Re: float("nan") in set or as key Albert Hopkins <marduk@letterboxes.org> - 2011-05-28 20:28 -0400
  Re: float("nan") in set or as key Erik Max Francis <max@alcyone.com> - 2011-05-28 17:44 -0700
  Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-05-29 02:25 +0000
    Re: float("nan") in set or as key Wolfgang Rohdewald <wolfgang@rohdewald.de> - 2011-05-29 10:27 +0200
      Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-05-29 14:41 +0000
        Re: float("nan") in set or as key MRAB <python@mrabarnett.plus.com> - 2011-05-29 18:44 +0100
          Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 18:27 +0000
        Re: float("nan") in set or as key Chris Angelico <rosuav@gmail.com> - 2011-05-30 03:50 +1000
        Re: float("nan") in set or as key Christian Heimes <lists@cheimes.de> - 2011-05-29 20:05 +0200
          Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 18:46 +0000
        Re: float("nan") in set or as key Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2011-05-29 20:36 -0700

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