[Python-Dev] decimal.py: == and != comparisons involving NaNs

[Mark Dickinson]

On Fri, Nov 13, 2009 at 6:18 PM, Adam Olsen <rhamph at gmail.com> wrote:
> On Mon, Nov 9, 2009 at 06:01, Mark Dickinson <dickinsm at gmail.com> wrote:
>> Well, when running in some form of 'non-stop' mode, where (quiet) NaN
>> results are supposed to be propagated to the end of a computation, you
>> certainly want equality comparisons with nan just to silently return false.
>> E.g., in code like:
>>
>> if x == 0:
>>    <deal with zero special case>
>> else:
>>    <usual algorithm>
>>
>> nans should just end up in the second branch, without the programmer
>> having had to think about it too hard.
>
> if x != 0:
>    <usual algorithm>
> else:
>    <deal with zero special case>
>
> nans should just end up in the first branch, without the programmer
> having had to think about it too hard.

And they do:  nan != 0 returns False.  Maybe I'm missing your point
here?

> IEEE 754 is intended for a very different context.  I don't think it
> makes sense to attempt literal conformance to it.

I disagree.  The decimal specification is tied closely to
IEEE 754 (the 2008 version), to the extent that even minor changes
made by the 754r working group were mirrored in the decimal
specification as it evolved;  it only stopped evolving after IEEE
754-2008 was complete.

IEEE 754-2008 also makes a point of targeting languages, rather than
just floating-point hardware;  to me it seems very much applicable
to decimal.py.

Mark