dictionary keys, __hash__, __cmp__

[Michael Hudson]

Jan-Erik Meyer-Lütgens  <python at meyer-luetgens.de> writes:

> Michael Hudson wrote:
> > Jan-Erik Meyer-Lütgens  <python at meyer-luetgens.de> writes:
> >>
> >>    3. "If a class does not define a __cmp__() method it
> >>        should not define a __hash__() operation either."
> >>
> >>
> >>Can I asume that:
> >>  -- I can savely ignore the 3rd statement, because python
> >>     falls back to cmp(id(obj1), id(obj2)), if __cmp__()
> >>     is not defined.
> > Don't understand.
> > 
> 
> Ok, let me ask the following question: What is the reason
> for that rule?

Well, the idea is that dictionary lookup is done by equality.  If you
don't define __cmp__ then equality is in fact identity (well, that's
very nearly true...) so the default implementation of __hash__ (based
on the pointer address) is as good as it can get (you have hash
equality iff you have object equality).

I think.

Cheers,
mwh

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