[issue15544] math.isnan fails with some Decimal NaNs

[Steven D'Aprano]

Steven D'Aprano added the comment:

On 03/08/12 22:06, Mark Dickinson wrote:
>
>> Decimal('snan').is_nan() just returns true and I am under the impression
>> that IEEE 754 specifies the same.
>
> Sure, but IEEE 754 also specifies that math.sqrt(<signalling nan>) should
> signal.  Since both math.sqrt and math.isnan are going through __float__,
> we can't keep everyone happy here.

Is it necessarily a given that math.isnan *must* go through __float__? If it 
were written in Python, it would be a simple matter of including

if isinstance(x, Decimal) and x.isnan(): return True

before the conversion to float. By I have no idea whether that is practical in 
the math module.

> The question for me is really what __float__ should do.  IEEE 754 doesn't
> help here, since it doesn't cover decimal floating-point<->  binary
> floating-point conversions.

Until such time that floats officially support snans, I think ValueError is 
the right behaviour.

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<http://bugs.python.org/issue15544>
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