[Python-ideas] Disallow orderring comparison to NaN

[Robert Kern]

On 4/28/11 5:10 PM, Guido van Rossum wrote:
> On Thu, Apr 28, 2011 at 12:25 PM, Alexander Belopolsky
> <alexander.belopolsky at gmail.com>  wrote:
>> I posted a patch implementing this proposal on the tracker:
>>
>> http://bugs.python.org/issue11949
>
> Interesting indeed! I'd like to hear from the numpy folks about this.

I'm personally -1, though mostly on general conservative principles. I'm sure 
there is some piece of code that will break, but I don't know how significant it 
would be.

I'm not sure that it solves a significant problem. I've never actually heard of 
anyone running into an infinite cycle due to NaNs, though a bit of Googling does 
suggest that it happens sometimes.

I don't think it really moves us closer to IEEE-754 compliance. The standard 
states (section 7. "Exceptions") "The default response to an exception shall be 
to proceed without a trap." Python only intermittently turns INVALID operations 
into exceptions, mostly just (-1.0)**0.5 and integer conversion (0/0.0 and x%0.0 
could be considered covered under the division by zero signal that *is* 
consistently turned into a Python exception). inf-inf, inf/inf, 0*inf, and 
inf%2.0, to give other examples of INVALID-signaling operations from the spec, 
all return a NaN without an exception. Given that we want to avoid exposing 
SIGFPE handlers for safety reasons, I think the status quo is a reasonable 
compromise interpretation of the spec.

-- 
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