[Python-ideas] Disallow orderring comparison to NaN

[Robert Kern]

On 4/29/11 4:30 PM, Alexander Belopolsky wrote:
> On Fri, Apr 29, 2011 at 4:23 PM, Robert Kern<robert.kern at gmail.com>  wrote:
> ..
>> (Section 7. "Exceptions") "The default response to an exception shall be to
>> proceed without a trap."
>
> I cannot find this phrase in my copy of IEEE Std 754-2008.  Instead, I
> see the following in section 7.1:
>
> "This clause also specifies default non-stop exception handling for
> exception signals, which is to deliver a default result, continue
> execution, and raise the corresponding status flag."

Ah. I have the 1985 version.

> Overall, it appears that in cases where InvalidOperation was
> anticipated, it was converted to some type of exception in Python.
> Exceptions to this rule seem to be an accident of implementation.

Well, for comparisons at least, it seems to have been anticipated, and returning 
a value was intentional. From the comments documenting float_richcompare() in 
floatobject.c:

/* Comparison is pretty much a nightmare.  When comparing float to float,
  * we do it as straightforwardly (and long-windedly) as conceivable, so
  * that, e.g., Python x == y delivers the same result as the platform
  * C x == y when x and/or y is a NaN.
...

I'm not sure there's any evidence that the other behaviors have not been 
anticipated or are accidents of implementation. The ambiguous inf operations are 
documented and doctested in Lib/test/ieee754.txt.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco