set using alternative hash function?

Austin Bingham austin.bingham at gmail.com
Thu Oct 15 14:50:20 CEST 2009


On Thu, Oct 15, 2009 at 2:23 PM, Diez B. Roggisch <deets at nospam.web.de> wrote:
> Austin Bingham wrote:
> This is a POV, but to to me, the set just deals with a very minimal
> protocol - hash-value & equality. Whatever you feed it, it has to cope with
> that. It strikes *me* as odd to ask for something else.

But I'm not really asking for anything that changes the paradigm of
how things currently work. All I need is the ability to say something
like this:

 s = set(hash_func = lambda x: hash(x.name))

If set could be de-hardcoded from using hash(), nothing else about how
it works would have to change. Or am I wrong on that? I see that you
mention equality in the protocol, but I don't know why a set would
need that if a) hash(x) == hash(y) --> x == y and b) hash function
must return a 32 bit value (which I think is the case)...so maybe I
misunderstand something.

I wonder...does the requirement of using hash() have to do with the
low-level implementation of set? That might explain the state of
things.

> The ideal solution would be to have several hash/eq-methods on your object,
> and somehow make the surroundings decide which to use. But there is no
> OO-pragma for that.

But why force objects to know about all of the wacky contexts in which
they might be used? Let objects be objects and hash-functions be
hash-functions. If someone wants a set where uniqueness is defined by
the number of occurrences of 'a' in an object's name, the object
should never have to know that...it should just have a name.

In any event, it sounds like set doesn't have any notion of switching
out its hash function, and that more or less answers my question.

Austin



More information about the Python-list mailing list