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

[Steven D'Aprano]

On Wed, 24 Mar 2010 08:51:36 pm Mark Dickinson wrote:
> On Wed, Mar 24, 2010 at 5:36 AM, Stephen J. Turnbull 
<stephen at xemacs.org> wrote:
> > Steven D'Aprano writes:
> >
> >  > 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.
>
> Right:  distinct nans (i.e., those with different id()) are treated
> as distinct set elements or dict keys.
>
> > I don't see how it can be so.  Aren't all of those entries garbage?
> > To compute a histogram of results for computations on a series of
> > cases would you not have to test each result for NaN-hood, then
> > hash on a proxy such as the string "Nan"?

Not necessarily -- you could merely ignore any key which is a NaN, or 
you could pass each key through this first:

def intern_nan(x, nan=float('nan')):
    if math.isnan(x):  return nan
    return x

thus ensuring that all NaN keys were the same NaN.

> So what alternative behaviour would you suggest, and how would you
> implement it?
[...]
> One alternative would be to prohibit putting nans into sets and dicts
> by making them unhashable;  I'm not sure what that would gain,
> though. And there would still be some unintuitive behaviour for
> containment testing of nans in lists.

I think that would be worse than the current situation. That would mean 
that dict[some_float] would *nearly always* succeed, but occasionally 
would fail. I can't see that being a good thing.


-- 
Steven D'Aprano