Rich Comparisons Gotcha

Rhamphoryncus rhamph at gmail.com
Mon Dec 8 22:10:53 CET 2008


On Dec 8, 1:04 pm, Robert Kern <robert.k... at gmail.com> wrote:
> Rhamphoryncus wrote:
> > On Dec 8, 11:54 am, Robert Kern <robert.k... at gmail.com> wrote:
> >> 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
> >>>> incoherent.
> >>> 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.
> >> Well, there are explicitly two kinds of NaNs: signalling NaNs and quiet NaNs, to
> >> accommodate both requirements. Additionally, there is significant flexibility in
> >> trapping the signals.
>
> > Right, but most of that's lower level.  By the time it reaches Python
> > we only care about quiet NaNs.
>
> No, signaling NaNs raise the exception that you are asking for. You're right
> that if you get a Python float object that is a NaN, it is probably going to be
> quiet, but signaling NaNs can affect Python in the way that you want.
>
> >>> Regarding the mythical IEEE 754, 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*.  It is C and
> >>> probably most other languages that turn that into false (as they want
> >>> a dummy value, not an error.)
> >>>http://groups.google.ca/group/sci.math.num-analysis/browse_thread/thr...
> >> Table 4 on page 9 of the standard is pretty clear on the subject. When the two
> >> operands are unordered, the operator == returns False. The standard defines how
> >> to do comparisons notionally; two operands can be "greater than", "less than",
> >> "equal" or "unordered". It then goes on to map these notional concepts to
> >> programming language boolean predicates.
>
> > Ahh, interesting.  Still though, does it give an explanation for such
> > behaviour, or use cases?  There must be some situation where blindly
> > returning false is enough benefit to trump screwing up sorting.
>
> Well, the standard was written in the days of Fortran. You didn't really have
> generic sorting routines. You *could* implement whatever ordering you wanted
> because you *had* to implement the ordering yourself. You didn't have to use a
> limited boolean predicate.
>
> Basically, the boolean predicates have to return either True or False. Neither
> one is really satisfactory, but that's the constraint you're under.

"We've always done it that way" is NOT a use case!  Certainly, it's a
factor, but it seems quite weak compared to the sort use case.

I suppose what I'm hoping for is an small example program (one or a
few functions) that needs the "always false" behaviour of NaN.



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