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

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  Re: float("nan") in set or as key Chris Angelico <rosuav@gmail.com> - 2011-05-29 10:32 +1000
    Re: float("nan") in set or as key Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 08:51 +0000

#6502 — Re: float("nan") in set or as key

On Sun, May 29, 2011 at 10:28 AM, Albert Hopkins <marduk@letterboxes.org> wrote:
> This is the same nan, so it is equal to itself.
>

Actually, they're not. But it's possible the dictionary uses an 'is'
check to save computation, and if one thing 'is' another, it is
assumed to equal it. That's true of most well-behaved objects, but nan
is not well-behaved :)

Chris Angelico

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#6519

On Sun, 29 May 2011 10:32:43 +1000, Chris Angelico wrote:

> On Sun, May 29, 2011 at 10:28 AM, Albert Hopkins
> <marduk@letterboxes.org> wrote:
>> This is the same nan, so it is equal to itself.
>>
>>
> Actually, they're not. But it's possible the dictionary uses an 'is'
> check to save computation, and if one thing 'is' another, it is assumed
> to equal it. That's true of most well-behaved objects, but nan is not
> well-behaved :)

*Exactly* correct.

NAN != NAN even if they are the same NAN, by design. This makes NANs ill-
behaved, but usefully so. Most (all?) Python built-ins assume that any 
object X is equal to itself, so they behave strangely with NANs.


-- 
Steven

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