set using alternative hash function?

[Chris Rebert]

On Thu, Oct 15, 2009 at 4:24 AM, Austin Bingham
<austin.bingham at gmail.com> wrote:
> If I understand things correctly, the set class uses hash()
> universally to calculate hash values for its elements. Is there a
> standard way to have set use a different function? Say I've got a
> collection of objects with names. I'd like to create a set of these
> objects where the hashing is done on these names. Using the __hash__
> function seems inelegant because it means I have to settle on one type
> of hashing for these objects all of the time, i.e. I can't create a
> set of them based on a different uniqueness criteria later. I'd like
> to create a set instance that uses, say, 'hash(x.name)' rather than
> 'hash(x)'.
>
> Is this possible? Am I just thinking about this problem the wrong way?
> Admittedly, I'm coming at this from a C++/STL perspective, so perhaps
> I'm just missing the obvious. Thanks for any help on this.

You could use wrapper objects that define an appropriate __hash__():

#*completely untested*
class HashWrapper(object):
    def __init__(self, obj, criteria):
        self._wrapee = obj
        self._criteria = criteria

    #override __hash__() / hash()
    def __hash__(self):
        return hash(self._criteria(self._wrapee))

    #proxying code
    def __getattr__(self, name):
        return getattr(self._wrapee, name)

    def __setattr__(self, name, val):
        setattr(self._wrapee, name, val)

#example
def name_of(obj):
    return obj.name

def name_and_serial_num(obj):
    return obj.name, obj.serial_number

no_same_names = set(HashWrapper(obj, name_of) for obj in some_collection)
no_same_name_and_serial = set(HashWrapper(obj, name_and_serial_num)
for obj in some_collection)

Cheers,
Chris
--
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