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

[Mark Dickinson]

On Wed, Mar 24, 2010 at 10:36 PM, Alexander Belopolsky
<alexander.belopolsky at gmail.com> wrote:
> On Wed, Mar 24, 2010 at 6:31 PM, Mark Dickinson <dickinsm at gmail.com> wrote:
> ..
>> Neither is necessary, because Python doesn't actually use == as the
>> equivalence relation for containment testing:  the actual equivalence
>> relation is:  x equivalent to y iff id(x) == id(y) or x == y.  This
>> restores the missing reflexivity (besides being a useful
>> optimization).
>
> No, it does not:
>
>>>> float('nan') in [float('nan')]
> False

Sure, but just think of it as having two different nans there.  (You
could imagine thinking of the id of the nan as part of the payload.)
There's no ideal solution here;  IMO, the compromise that currently
exists is an acceptable one.

Mark