PEP 218 Re: ANN: set-0.1 module available

[Gonçalo Rodrigues]

On 18 May 2002 17:15:29 +0200, Bernhard Herzog <bh at intevation.de> wrote:

>Erik Max Francis <max at alcyone.com> writes:
>
>> That's not the half of it.  Mutable objects can dynamically change
>> state, completely outside the control of the container.  So the hard
>> question is:  What happens when a mutable object changes its value, such
>> that it effects the arrangement of the set?  How does the container get
>> notified of this change (which as far as I know there is no standard way
>> to test in Python), and what happens when a value collision occurs
>> within the set?  Does one object get disposed?  If so, which one?
>
>It seems to me that the most sensible solution would be to deal with it
>like a dictionary deals with mutable keys: By not caring about whether
>an object is mutable or not (there is no way to determine this anyway)
>but relying on a certain behavior regarding hash values instead.
>
>If an object is stored in a dict as a key, the only things that matter
>are that
>
>1. its hash value doesn't change as long as the object is used as a key.

Doesn't this mean, in practice, that it is immutable? To me, this
requirement is just shifting responsabilities from one place to the
other and not really solving the problem.

>
>2. equal objects have equal hash values
>
>There may be some more subtle requirements. I'm not sure what happens
>when two objects that have the same hash value but do not compare eqal
>at first later become equal, for instance.

I believe 2. is more stringent:

equal objects iff equal hash values

>
>   Bernhard

Best regards,
Gonçalo Rodrigues