On Fri, Jun 19, 2009 at 12:02 PM, Gustavo Narea<m...@gustavonarea.net> wrote: > Hello, everyone. > > I've noticed that if I have a class with so-called "rich comparison" > methods > (__eq__, __ne__, etc.), when its instances are included in a set, > set.__contains__/__eq__ won't call the .__eq__ method of the elements > and thus > the code below: > """ > obj1 = RichComparisonClass() > obj2 = RichComparisonClass() > > set1 = set([obj1]) > set2 = set([obj2]) > > if obj1 == obj2: > print "Objects 1 and 2 are equivalent" > else: > print "Objects 1 and 2 aren't equivalent" > > if set1 == set2: > print "Sets 1 and 2 are equivalent" > else: > print "Sets 1 and 2 aren't equivalent" > """ > > Would output: > """ > Objects 1 and 2 are equivalent > Sets 1 and 2 aren't equivalent > """ > > instead of: > """ > Objects 1 and 2 are equivalent > Sets 1 and 2 are equivalent > """ > > How can I work around this? The only solution that comes to my mind is
Note that sets are dict-based and thus use hash tables internally. Thus, you must implement a sensible __hash__ method. The default __hash__ provided by class `object` uses the object ID for the hash. Since id(x) == id(y) iff x is y, and hash(x) != hash(y) implies x != y, If you don't implement __hash__, object's implementation causes every distinct object of your type to be considered unequal a priori, so it doesn't bother to check any further by calling the comparison methods. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
