[Python-3000] Need help completing ABC pep

[Guido van Rossum]

On 4/21/07, Nick Coghlan <ncoghlan at gmail.com> wrote:
> Guido van Rossum wrote:
> > I do see your point though: object.__hash__ makes it possible to use
> > all sorts of objects as dict keys by default, as long as they don't
> > implement __eq__. But perhaps this is not such a good idea and we
> > should just get rid of it and add __hash__ back to specific objects
> > that are useful to use as keys? (E.g. classes but not iterators, to
> > take some extreme examples.)
>
> We've tried to do this before - the point where we got stuck was the
> fact that Jython needed a separate location to store the default hash
> method (unlike CPython, Jython's id() and object.__hash__() do different
> things), and we couldn't come up with a reasonable location for it.

I don't understand. Why does the default hash method need to be stored
in a location? Why couldn't it be a method on PyObj or whatever their
base object it? Or are you talking about a place to store the hash
*value*? But why would that need to be stored?

> I agree that moving the default method from object to Hashable doesn't
> eliminate the errors that arise from redefining __eq__ without
> redefining __hash__, but at least it restricts them to those classes
> which have Hashable somewhere in their MRO.

Actually it would significantly reduce the errors because
Hashable.__hash__ is abstract, *forcing* you to redefine it. My
concern is more with the amount of broken code if we remove
object.__hash__.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)