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

[Pierre B.]

Mark Dickinson <dickinsm <at> gmail.com> writes:

> 
> On Thu, Mar 25, 2010 at 12:39 PM, Jesus Cea <jcea <at> jcea.es> wrote:
> >
> > But IEEE 754 was created by pretty clever guys and sure they had a
> > reason for define things in the way they are. Probably we are missing
> > something.
> 
> Well, if we are, then nobody seems to know what!  See the Bertrand
> Meyer blog post that was linked to up-thread.

The missing part, IMO, is that allowing a given NaN value to compare equal to
itself only pushes the problem up one level. Any single operation yielding a
NaN will still be unequal to itself. That is, under what is being proposed,
with a function func() returning the same result of some calculation:

x = func()
s1 = (x)
print x in s1
print func() in s1

This would print True and False, even though func() is perfoming the same
calculation and thus logically returning the same NaN.

I think the IEEE NaN represent the fact that you have a number of an undefined
set, but it doesn't specify which.

The only way out, IMO, is to make *all* NaN comparisons yield False, but
identity yield true. No interning necessary. At most, you could make the
identity function return False for the different types of NaN.