class MySet(frozenset):

    def __new__(cls, vals):
        print("__new__(%r, %r)" % (cls, vals, ))
        return frozenset.__new__(cls, vals)

    def __init__(self, vals):
        print("__init__(%r, %r)" % (self, vals, ))


x = MySet((3, 4))
y = MySet((4, 2))
z = x.difference(y)
print(type(z), repr(z))