What is the correct (or best) way to subclass the Python set class, adding a new instance variable?

Question

I\'m implementing an object that is almost identical to a set, but requires an extra instance variable, so I am subclassing the built-in set object. What is the best way to

Answer 1

set1 | set2 is an operation that won't modify either existing set, but return a new set instead. The new set is created and returned. There is no way to make it automatically copy arbritary attributes from one or both of the sets to the newly created set, without customizing the | operator yourself by defining the __or__ method.

class MySet(set):
    def __init__(self, *args, **kwds):
        super(MySet, self).__init__(*args, **kwds)
        self.foo = 'nothing'
    def __or__(self, other):
        result = super(MySet, self).__or__(other)
        result.foo = self.foo + "|" + other.foo
        return result

r = MySet('abc')
r.foo = 'bar'
s = MySet('cde')
s.foo = 'baz'

t = r | s

print r, s, t
print r.foo, s.foo, t.foo

Prints:

MySet(['a', 'c', 'b']) MySet(['c', 'e', 'd']) MySet(['a', 'c', 'b', 'e', 'd'])
bar baz bar|baz