[Python-ideas] Why is nan != nan?

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On Fri, 26 Mar 2010 15:01:46 +1300
Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

> The more I think about it, the more I think that the
> only reason for needing NaNs in the first place is if
> you don't have, or don't want to use, an exception
> mechanism.
> 
> As Meyer points out, treating NaN == NaN as false is
> not fundamentally any more correct than treating it
> as true. One interpretation or the other might be
> appropriate in a particular algorithm, but this needs
> to be considered on a case-by-case basis.

[In a language or a given case where yielding a NaN does not raise an exception from scratch.]

It seems to me there are various comparison cases (esp. in an OO language, case 2):
-1- Comparing (any) Nan to non_NaN should return False.
	a = <expr yielding a NaN>
	b = 1
	a == b 	# ==> False
-2- Comparing a given NaN with itself should return True.
	a = <expr yielding a NaN>
	b = a
	a == b 	# ==> True
-3- Comparing various NaNs is the undefined case. Actually, I think it's the only case where a theoretical NaB makes sense. In my opinion, as Greg says, the python way would here be raising an exception. But only in that case.


Denis
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