Base your set on list : Your Set « Collections « Python Tutorial

1. Python Tutorial
2. Collections
3. Your Set

class Set(list):
    def __init__(self, value = []):
        list.__init__([])
        self.concat(value)

    def intersect(self, other):
        res = []
        for x in self:
            if x in other:
                res.append(x)
        return Set(res)

    def union(self, other):
        res = Set(self)
        res.concat(other)
        return res

    def concat(self, value):
        for x in value:
            if not x in self:
                self.append(x)

    def __and__(self, other): return self.intersect(other)
    def __or__(self, other):  return self.union(other)
    def __repr__(self):       return 'Set:' + list.__repr__(self)

x = Set([1,3,5,7])
y = Set([2,1,4,5,6])
print x, y, len(x)
print x.intersect(y), y.union(x)
print x & y, x | y
x.reverse(  ); print x