On 26.08.2021 12:15, Steven D'Aprano wrote:
> On Thu, Aug 26, 2021 at 11:05:01AM +0200, Marc-Andre Lemburg wrote:
>
>> Oh, good point. I was under the impression that NAN is handled
>> as a singleton.
>
> There are 4503599627370496 distinct quiet NANs (plus about the same
> signalling NANs). So it would need to be 4-quadrillion-ton :-)
>
> (If anyone is concerned about the large number of NANs, it's less than
> 0.05% of the total number of floats.)
>
> Back in the mid-80s, Apple's floating point library, SANE, distinguished
> different classes of error with distinct NANs. Few systems have followed
> that lead, but each NAN still has 51 bits available for a diagnostic
> code, plus the sign bit. While Python itself only generates a single NAN
> value, if you are receiving data from outside sources it could contain
> NANs with distinct payloads.
>
> The IEEE-754 standard doesn't mandate that NANs preserve the payload,
> but it does recommend it. We shouldn't gratuitously discard that
> information. It could be meaningful to whoever is generating the data.

Fair enough. Would it then make sense to at least have all possible
NAN objects compare equal, treating the extra error information as an
attribute value rather than a distinct value and perhaps exposing this
as such ?

I'm after the "practicality beats purity" here. The math.isnan() test
doesn't work well in practice, since you'd have to iterate over all
sequence members and call that test function, which is expensive when
done in Python.

--
Marc-Andre Lemburg
eGenix.com

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