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

[Mark Dickinson]

On Wed, Mar 24, 2010 at 11:47 AM, Nick Coghlan
> Interning NaN certainly seems like it should be sufficient to eliminate
> the set/dict membership weirdness.
>
> That is, make it so that the first two lines of the following return
> True, while the latter two lines continue to return False:
>
>>>> float("nan") is float("nan")
> False
>>>> dec("nan") is dec("nan")
> False
>>>> float("nan") == float("nan")
> False
>>>> dec("nan") == dec("nan")
> False

Yes;  that could be done.  Though as Steven points out, not all NaNs
are equivalent (possibility of different payloads and different
signs), so float nans with different underlying bit patterns, and
Decimal nans with different string representations, would ideally be
interned separately.  For floats it might be possible to get away with
pretending that there's only one nan.  For decimal, I don't think
that's true, since the payload and sign are part of the standard, and
are very visible (e.g. in the repr of the nan).

The obvious way to do this nan interning for floats would be to put
the interning code into PyFloat_FromDouble.  I'm not sure whether this
would be worth the cost in terms of added code (and possibly reduced
performance, since the nan check would be done every time a float was
returned), but I'd be willing to review a patch.

Mark