> > > This code
> > >
> > > class X:
> > > def __cmp__(self, other):
> > > return 1
> > >
> > > c1 = X()
> > >
> > > d = {}
> > > d[c1] = None
> > >
> > > raises a TypeError, unhashable type, since the class does
> > > not define a __hash__ method.
> > >
> > > This error is no longer raised when X derives from object.
> > > Bug (or feature)?
> >
> > It's something in between. :-(
> >
> > I've been struggling with this since 2.2 and not found a good
> > solution. Built-in mutable types like list and dict deal with this by
> > having an explicit tp_hash slot that raises an exception.
>
> Why doesn't the default implementation raise a TypeError (or be NULL),
> and the immutable types override this?
The default implementation of __hash__ must match the default
implementation of __cmp__ (and rich comparisons, __eq__ etc.). So the
default implementation cannot raise an exception, because objects are
defined to be immutable by default. (Maybe this was a mistake, but
it's not so easy to change without causing backwards
incompatibilities.)
> > Can you add a SF entry and assign it to me? Any insight you might
> > have in the matter would be appreciated.
>
> Sure.
Thanks.
--Guido van Rossum (home page: http://www.python.org/~guido/)
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