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Tue Mar 23 23:00:30 CET 2010 · https://mail.python.org/pipermail/python-dev/2010-March/098771.html

On Wed, 24 Mar 2010 05:04:37 am Mark Dickinson wrote:
> On Tue, Mar 23, 2010 at 5:48 PM, Adam Olsen <rhamph at gmail.com> wrote:
> > a = Decimal('nan')
> > a != a
> >
> > They don't follow the behaviour required for being hashable.
>
> What's this required behaviour?  The only rule I'm aware of is that
> if a == b then hash(a) == hash(b).  That's not violated here.
>
> Note that containment tests check identity before equality, so
> there's no problem with putting (float) nans in sets or dicts:
> >>> x = float('nan')
> >>> s = {x}
> >>> x in s
> True

As usual though, NANs are unintuitive:

>>> d = {float('nan'): 1}
>>> d[float('nan')] = 2
>>> d
{nan: 1, nan: 2}

I suspect that's a feature, not a bug.

-- 
Steven D'Aprano

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[Python-Dev] Mixing float and Decimal -- thread reboot