That's definitely a workable solution, but it still rubs me the wrong way. The uniqueness criteria of a set seems, to me, like a property of the set, whereas the python model forces it onto each set element.
Another issue I have with the HashWrapper approach is its space requirements. Logically, what I'm asking to do is switch out a single function reference (i.e. to point at get_name() rather than hash()), but in practice I'm forced to instantiate a new object for each of my set members. On a large set, this could be disastrous. Don't get me wrong...your solution is a good one, but it's just not what I am looking for. Austin On Thu, Oct 15, 2009 at 1:36 PM, Chris Rebert <c...@rebertia.com> wrote: > On Thu, Oct 15, 2009 at 4:24 AM, Austin Bingham > <austin.bing...@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 > -- > http://blog.rebertia.com > -- http://mail.python.org/mailman/listinfo/python-list
