[Python-Dev] Python's C interface for types

[Nick Maclaren]

"Jim Jewett" <jimjjewett at gmail.com> wrote:
> 
> >> For 0: hash(+0.0)==hash(-0.0)==hash(0)=hash(0L)=0
> 
> > Unfortunately, that assumes that equality is transitive.
> 
> No, but the (transitively closed set of equivalent objects) must have
> the same hash.  ...

Er, how do you have a transitive closure for a non-transitive operation?

I really do mean that quite a lot of floating-point bells and whistles
are non-transitive.  The only one most people will have come across is
IEEE NaNs, where 'a is b' does not imply 'a == b', but there are a
lot of others (and have been since time immemorial).  I don't THINK
that IEEE 754R decimal introduces any, though I am not prepared to
bet on it.

> > let us say that I am implementing a special function and want to
> > distinguish -0.0 and +0.0.  Why can't I use a dictionary?
> 
> Because they are equal.  They aren't identical, but they are equal.

You have missed my point, which is extended floating-points effectively
downgrade the status of the purely numeric comparisons, and therefore
introduce a reasonable requirement for using a tighter match.  Note
that I am merely commenting that this needs bearing in mind, and NOT
that anything should be changed.

> >    >>> a = float("+0.0")
> >    >>> b = float("-0.0")
> >    >>> print a, b
> >    0.0 -0.0
> 
> With the standard windows distribution, I get just
> 
>     0.0 0.0

Watch that space :-)  Expect it to change.


Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679