In python - the operator which a set uses for test if an object is in the set

set uses __hash__ for comparison. Override that, and you'll be good:

class MyClass(object):
    def __init__(self, s):
        self.s = s
    def __cmp__(self, other):
        return cmp(self.s, other.s)
    def __hash__(self):
        return hash(self.s) # Use default hash for 'self.s'

instance1, instance2 = MyClass("a"), MyClass("a")
instance3 = MyClass("b")

print instance2==instance1             # True
print instance2 in [instance1]         # True
print instance2 in set([instance1])    # True