[Python-Dev] Mixing float and Decimal -- thread reboot (original) (raw)

Steven D'Aprano steve at pearwood.info
Wed Mar 24 12:14:39 CET 2010

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

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