Numeric comparison anomaly
Gary Herron
gherron at islandtraining.com
Thu Feb 20 22:30:28 CET 2003
On Thursday 20 February 2003 12:38 pm, sismex01 at hebmex.com wrote:
> > From: Piet van Oostrum [mailto:piet at cs.uu.nl]
> > Sent: Thursday, February 20, 2003 4:56 AM
> >
> > >>>>> Gerrit Holl <gerrit at nl.linux.org> (GH) schreef:
> >
> > GH> You can use the __cmp__ overloader:
> >
> > GH> 21 >>> class A:
> > GH> 21 ... def __cmp__(self, other):
> > GH> 21 ... return 1
> > GH> 21 ...
> > GH> 22 >>> A() > 5
> > GH> True
> > GH> 23 >>> A() < 9
> > GH> False
> > GH> 24 >>> A() >= 3
> > GH> True
> >
> > >>> inf = A()
> > >>> inf > inf
> >
> > True
> >
> > >>> inf == inf
> >
> > False
>
> This is correct, or wrong?
>
> -gustavo
This is correct of course. By having __cmp__ return 1 *always*, you
are saying inf is greater than *anything* else without regard to what
the other thing is. (Note that your __cmp__ does not even look at the
second argument.) It will never compare equal to anything (__cmp__
would need to return a zero for that).
Thus
inf > ...any python object...
returns True because __cmp__ returns 1 (meaning "is greater than") and
inf == ...any python object...
returns False because __cmp__ returns 1 (meaning "is greater than" --
not "equal to").
Did you perhaps miss that __cmp__ must return one of three casses:
negative integer: meaning "self is less than other"
0: meaning "self is equal to other"
positive integer: meaning "self is greater than other"
Gary Herron
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