[Python-ideas] checking for identity before comparing built-in objects

[Steven D'Aprano]

On Mon, Oct 08, 2012 at 11:44:12PM -0700, Guido van Rossum wrote:
> This smells like a bug in the != operator, it seems to fall back to not ==
> which it didn't used to. More later.....

I'm pretty sure the behaviour is correct. When I get home this evening, 
I will check my copy of the Standard Apple Numerics manual (one of the 
first IEEE 754 compliant systems). In the meantime, I quote from 

"What Every Computer Scientist Should Know About Floating-Point 
Arithmetic"

"Since comparing a NaN to a number with <, ≤, >, ≥, or = (but not ≠) 
always returns false..."

(Admittedly it doesn't specifically state the case of comparing a NAN 
with a NAN.)

http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html


-- 
Steven