On Wed, 15 Dec 2004 23:38:04 -0500, Adam DePrince <[EMAIL PROTECTED]> wrote:
>On Wed, 2004-12-15 at 10:26, Jp Calderone wrote:
> > On Wed, 15 Dec 2004 14:18:21 GMT, Roel Schroeven <[EMAIL PROTECTED]> wrote:
> > >Antoon Pardon wrote:
> > > > Op 2004-12-15, Fredrik Lundh schreef <[EMAIL PROTECTED]>:
> > > >>sorry, but I don't understand your reply at all. are you saying that
> > > >>dictionaries
> > > >>could support mutable keys (e.g lists) by making a copy of the key?
> > > >>how would
> > > >>such a dictionary pick up changes to the original key object? (I'm
> > > >>talking about
> > > >>the key stored in the dictionary, not the key you're using to look
> > > >>things up).
> > > >
> > > >
> > > > You want to mutate a key that is within a dictionary?
> > >
> > > No, we don't want to mutate it; as far as I know, that is exactly the
> > > reason why dictionaries don't support mutable keys.
> >
> > Dictionaries support mutable keys just find. What they don't
> > support is unhashable keys.
> >
> > For some objects, this is an important distinction: lists are
> > mutable but not hashable.
> >
> > For other objects, it is not: instances of user defined classes
> > are mutable and hashable. This is handy since the default hash is
> > based on identity instead of the values of attributes.
> >
> > Mutating an object in a dictionary is completely reasonable. What
> > is unreasonable is wanting to make a change that would change its
> > hash value.
>
> And how exactly do you propose to mutate an object without changing its
> hash value?
>
> [snip - argument predicated on incorrect definition of the word "same"]
>
It is often useful to treat two objects as "the same" if they are
/the same/ object. In other words, identity can be a useful hash value.
The attributes or other values (as in the case of an int or list, where
the object `is' the value instead of `has' the value) don't matter.
Since no mutation you can make to an object will change its identity,
when identity is used as a hash value, any mutation is safe with respect
to dictionaries.
Memos, caches, and registries often benefit from this (although since
many Python objects aren't hashable, they sometimes need to skip
__hash__ and manually use id() to compute a suitable hash value).
Jp
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