On Mon, May 30, 2011 at 10:55 AM, Carl Banks <pavlovevide...@gmail.com> wrote: > If exceptions had commonly existed in that environment there's no chance they > would have chosen that behavior; comparison against NaN (or any operation > with NaN) would have signaled a floating point exception. That is the > correct way to handle exceptional conditions. > > The only reason to keep NaN's current behavior is to adhere to IEEE, but > given that Python has trailblazed a path of correcting arcane mathematical > behavior, I definitely see an argument that Python should do the same for > NaN, and if it were done Python would be a better language.
If you're going to change behaviour, why have a floating point value called "nan" at all? Other than being a title for one's grandmother, what meaning does that string have, and why should it be able to be cast as floating point? Lifting from http://en.wikipedia.org/wiki/NaN a list of things that can return a NaN (I've removed non-ASCII characters from this snippet): * Operations with a NaN as at least one operand. (you need to bootstrap that somehow, so we can ignore this - it just means that nan+1 = nan) * The divisions 0/0 and infinity/infinity * The multiplications 0*infinity and infinity*0 * The additions +inf + (-inf), (-inf) + +inf and equivalent subtractions * The standard pow function and the integer exponent pown function define 0**0, 1**inf, and inf**0 as 1. * The powr function define all three indeterminate forms as invalid operations and so returns NaN. * The square root of a negative number. * The logarithm of a negative number * The inverse sine or cosine of a number that is less than -1 or greater than +1. Rather than having comparisons with NaN trigger exceptions, wouldn't it be much cleaner to have all these operations trigger exceptions? And, I would guess that they probably already do. NaN has an additional use in that it can be used like a "null pointer"; a floating-point variable can store 1.0, or 0.000000000005, or "no there's no value that I'm storing in this variable". Since a Python variable can contain None instead of a float, this use is unnecessary too. So, apart from float("nan"), are there actually any places where real production code has to handle NaN? I was unable to get a nan by any of the above methods, except for operations involving inf; for instance, float("inf")-float("inf") == nan. All the others raised an exception rather than return nan. Chris Angelico -- http://mail.python.org/mailman/listinfo/python-list
