Rich Comparisons Gotcha
steven at REMOVE.THIS.cybersource.com.au
Tue Dec 9 03:44:43 CET 2008
On Mon, 08 Dec 2008 10:20:56 -0800, Rhamphoryncus wrote:
> On Dec 7, 4:20 pm, Steven D'Aprano <st... at REMOVE-THIS-
> cybersource.com.au> wrote:
>> On Sun, 07 Dec 2008 15:32:53 -0600, Robert Kern wrote:
>> > Rasmus Fogh wrote:
>> >> Current behaviour is both inconsistent and counterintuitive, as
>> >> these examples show.
>> >>>>> x = float('NaN')
>> >>>>> x == x
>> >> False
>> > Blame IEEE for that one. Rich comparisons have nothing to do with
>> > that one.
>> There is nothing to blame them for. This is the correct behaviour. NaNs
>> should *not* compare equal to themselves, that's mathematically
> Mathematically, NaNs shouldn't be comparable at all. They should raise
> an exception when compared. In fact, they should raise an exception
> when *created*. But that's not what we want. What we want is a dummy
> value that silently plods through our calculations. For a dummy value
> it seems a lot more sense to pick an arbitrary yet consistent sort order
> (I suggest just above -Inf), rather than quietly screwing up the sort.
> Regarding the mythical IEEE 754,
It's hardly mythical.
> although it's extremely rare to find
> quotations, I have one on just this subject. And it does NOT say "x ==
> NaN gives false". It says it gives *unordered*.
Unordered means that none of the following is true:
x > NaN
x < NaN
x == NaN
It doesn't mean that comparing a NaN with something else is an error.
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