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

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On 2011-06-03, Nobody <nobody@nowhere.com> wrote:

>>> This would produce the same end result as raising an exception
>>> immediately, but would reduce the number of isnan() tests.
>> 
>> I've never found the number of isnan() checks in my code to be an
>> issue -- there just arent that many of them, and when they are there,
>> it provides an easy to read and easy to maintain way to handle things.
>
> I think that you misunderstood. What I was saying here was that, if you
> wanted exception-on-NaN behaviour from Python, the interpreter wouldn't
> need to call isnan() on every value received from the FPU, but rely upon
> NaN-propagation and only call it at places where a NaN might disappear
> (e.g. comparisons).

Ideed, I did misunderstand.  I thought you were talking about a
the value of reducing the number of isnan() tests in user application
code.

>>> I mean undefined, in the sense that 0/0 is undefined
>> 
>> But 0.0/0.0 _is_ defined.  It's NaN.  ;)
>
> Mathematically, it's undefined.

True, but we must be careful not to confuse math and scientific
calculation using fixed-length binary floting point.

>> IMHO, that's a bug.  IEEE-754 states explicit that 0.0/0.0 is NaN.
>> Pythons claims it implements IEEE-754.  Python got it wrong.
>
> But then IEEE-754 considers integers and floats to be completely
> different beasts, while Python makes some effort to maintain a
> unified "numeric" interface. If you really want IEEE-754
> to-the-letter, that's undesirable, although I'd question the choice
> of Python in such situations.

Python's minor issues with IEEE-754 are far outweighed by advantages
in other areas. :)

>>> If anything, it has proven to be a major nuisance. It takes a lot of
>>> effort to create (or even specify) code which does the right thing in
>>> the presence of NaNs.
>> 
>> That's not been my experience.  NaNs save a _huge_ amount of effort
>> compared to having to pass value+status info around throughout
>> complex caclulations.
>
> That's what exceptions are for. NaNs probably save a huge amount of
> effort in languages which lack exceptions, but that isn't applicable
> to Python.

How do you obtain using exceptions a behavior that's the same as with
quiet NaNs?

>>>> The correct answer to "nan == nan" is to raise an exception,
>>>> because you have asked a question for which the answer is nether True
>>>> nor False.
>>> 
>>> Wrong.
>>
>> That's overstating it. 

It was an attempt to illustate the overstatement to which it was a
reply.

-- 
Grant Edwards               grant.b.edwards        Yow! You should all JUMP
                                  at               UP AND DOWN for TWO HOURS
                              gmail.com            while I decide on a NEW
                                                   CAREER!!

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float("nan") in set or as key MRAB <python@mrabarnett.plus.com> - 2011-05-29 00:41 +0100
  Re: float("nan") in set or as key Erik Max Francis <max@alcyone.com> - 2011-05-28 17:16 -0700
  Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 00:26 +0000
  Re: float("nan") in set or as key Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-05-29 13:04 +1200
    Re: float("nan") in set or as key John Nagle <nagle@animats.com> - 2011-05-28 23:12 -0700
      Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 10:29 +0000
        Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-05-29 22:19 +0100
          Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 23:31 +0000
            Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-01 21:41 +0100
              Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-02 09:54 +0000
                Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-02 13:05 +0000
                Re: float("nan") in set or as key Robert Kern <robert.kern@gmail.com> - 2011-06-02 12:04 -0500
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-02 21:47 +0100
                Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-03 14:52 +0000
                Re: float("nan") in set or as key Chris Torek <nospam@torek.net> - 2011-06-03 17:52 +0000
                Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-06 13:54 +0000
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-04 00:29 +0100
                Re: float("nan") in set or as key Chris Angelico <rosuav@gmail.com> - 2011-06-04 09:51 +1000
                Re: float("nan") in set or as key rusi <rustompmody@gmail.com> - 2011-06-04 00:52 -0700
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-04 20:29 +0100
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-05 07:21 +0000
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-05 19:15 +0100
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-06 00:55 +0000
                Re: float("nan") in set or as key Nobody <nobody@nowhere.com> - 2011-06-06 23:14 +0100
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-06 23:44 +0000
                Re: float("nan") in set or as key Chris Angelico <rosuav@gmail.com> - 2011-06-07 11:00 +1000
                Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-06 14:03 +0000
                Re: float("nan") in set or as key Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-06-03 11:17 +1200
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-03 04:23 +0000
                Re: float("nan") in set or as key Chris Angelico <rosuav@gmail.com> - 2011-06-03 14:35 +1000
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-03 05:59 +0000
                Re: float("nan") in set or as key Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-06-04 12:14 +1200
                Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-04 02:21 +0000
                Re: float("nan") in set or as key Erik Max Francis <max@alcyone.com> - 2011-06-05 00:27 -0700
          Re: float("nan") in set or as key Grant Edwards <invalid@invalid.invalid> - 2011-06-01 21:01 +0000
      Re: float("nan") in set or as key Chris Torek <nospam@torek.net> - 2011-05-30 00:02 +0000
  Re: float("nan") in set or as key Raymond Hettinger <python@rcn.com> - 2011-05-29 21:49 -0700

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