[Python-Dev] Rich comparisons (original) (raw)

Michael Hudson mwh at python.net
Mon Mar 29 09:21:01 EST 2004

Edward Loper <edloper at gradient.cis.upenn.edu> writes:

Michael Hudson wrote:

>>> float('nan') nan >>> == False This means that 'nan' is no longer a well-behaved dictionary key: >>> x = {float('nan'):0} >>> x[float('nan')] = 1 >>> print x {nan: 0, nan: 1} Even worse, we get different behavior if we use the "same copy" of nan: >>> nan = float('nan') >>> x = {nan:0} >>> x[nan] = 1 >>> print x {nan: 1}

Gah. Still, I think this is more a theoretical objection than anything else?

BTW, with 2.3 on OS X:

{1e3000/1e3000:1}[0] 1 {0:2}[1e3000/1e3000] 2

So your 'no longer' isn't really valid :-)

If we really want nan==nan to be false, then it seems like we have to say that nan is unhashable. I'm also disturbed by the fact that cmp() has something different to say about their equality:

>>> cmp(float('nan'), float('nan')) 0

Well, yah. cmp() assumes a total ordering. If there just isn't one, what can we do?

I have at no point claimed that I have given Python 2.4 a coherent ieee 754 floating point story. All I have tried to do is /reduce/ the level of surprise knocking around, and I think I've succeeded. If someone (not me!) has the time and energy to do a 'proper job' (and I'd expect working out what that means to be the hard part), then you have my support and pre-emptive thanks.

Cheers, mwh

-- MARVIN: Oh dear, I think you'll find reality's on the blink again. -- The Hitch-Hikers Guide to the Galaxy, Episode 12

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