[Python-Dev] Why is nan != nan? (original) (raw)

Glenn Linderman v+python at g.nevcal.com
Fri Mar 26 02:56:00 CET 2010

On 3/25/2010 4:14 PM, Guido van Rossum wrote:

On Thu, Mar 25, 2010 at 12:31 PM, Glenn Linderman<v+python at g.nevcal.com> wrote:

It is my understand that even bit-for-bit identical NaN values will compare unequal according to IEEE 754 rules.

I would have no problem with Python interning each encountered NaN value, to avoid having bit-for-bit identical NaN values with different Python IDs, but having them compare equal seems inappropriate. Let's please not intern NaNs. Interning is for performance. If you have enough NaNs to affect your performance I think you have bigger worries!

I'm OK with interning NaNs, and I'm OK with not interning NaNs. But I would much prefer to see Python conform to the IEEE 754 for float, and the Decimal standard for Decimal, where at all possible, including keeping NaNs not comparing equal to anything, themselves included.

And retaining a mode where Decimal is standoffish, and doesn't accept non-Decimal comparisons or arithmetic seems like a big win for program correctness to me.

Glenn

More information about the Python-Dev mailing list