[Python-Dev] Dicts are broken ...

Nick Maclaren nmm1 at cus.cam.ac.uk
Fri Aug 4 20:59:34 CEST 2006

Michael Hudson <mwh at python.net> wrote:
> I'd say it's more to do with __eq__.  It's a strange __eq__ method
> that raises an Exception, IMHO.

Not entirely.  Any type that supports invalid values (e.g. IEEE 754)
and is safe against losing the invalid state by accident needs to
raise an exception on A == B.  IEEE 754 is not safe.

> Please do realize that the motivation for this change was hours and
> hours of tortous debugging caused by a buggy __eq__ method making keys
> "impossibly" seem to not be in dictionaries.

Quite.  Been there - been caught by that.  It is a catastrophic (but
very common) misdesign to conflate failure and the answer "no".
There is a fundamental flaw of that nature in card-based banking,
that I pointed out was insoluble to the Grid people, and then got
caught by just a month later!

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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