[Python-Dev] Python's C interface for types (original) (raw)

Nick Maclaren nmm1 at cus.cam.ac.uk
Thu Feb 1 10:41:59 CET 2007

"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

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