float("nan") in set or as key

[Grant Edwards]

On 2011-05-29, Nobody <nobody at nowhere.com> wrote:
> On Sun, 29 May 2011 10:29:28 +0000, Steven D'Aprano wrote:
>
>>> 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. There's a good argument to be made for raising
> an exception. Bear in mind that an exception is not necessarily an
> error, just an "exceptional" condition.
>
>> The correct answer to "nan == nan" is False, they are not equal.
>
> There is no correct answer to "nan == nan".

For those of us who have to deal with the real world (that means
complying with IEEE-754), there _is_ a correct answer.  IIRC, the IEEE
standard requires nan == nan is false, and nan != nan is true.

That said, I don't remember what the other comparisons are supposed to
do...

> Defining it to be false is just the "least wrong" answer.
>
> Arguably, "nan != nan" should also be false, but that would violate
> the invariant "(x != y) == !(x == y)".

And it would violate the IEEE standard.  IEEE-754 has it's warts, but
we're far better off than we were with dozens of incompatible,
undocumented, vendor-specific schemes (most of which had more warts
than IEEE-754).

-- 
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