[Python-Dev] Not-a-Number

[Robert Kern]

On 4/29/11 1:35 AM, Nick Coghlan wrote:
> On Fri, Apr 29, 2011 at 3:28 PM, Steven D'Aprano<steve at pearwood.info>  wrote:
>> Robert Kern wrote:
>>> Actually, Python treats all NaNs as quiet NaNs and never signalling NaNs.
>>
>> Sorry, did I get that backwards? I thought it was signalling NANs that cause
>> a signal (in Python terms, an exception)?
>>
>> If I do x = 0.0/0 I get an exception instead of a NAN. Hence a signalling
>> NAN.
>
> Aside from the divide-by-zero case, we treat NaNs as quiet NaNs.

And in fact, 0.0/0.0 is covered by the more general rule that x/0.0 raises 
ZeroDivisionError, not a rule that converts IEEE-754 INVALID exceptions into 
Python exceptions. Other operations that produce a NaN and issue an IEEE-754 
INVALID signal do not raise a Python exception.

But that's not the difference between signalling NaNs and quiet NaNs. A 
signalling NaN is one that when it is used as an *input* to an operation, it 
issues an INVALID signal, not whether a signal is issued when it is the *output* 
of an operation.

-- 
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