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

[Alexander Belopolsky]

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


It would if NaNs were always interned, but they are not.