>>> class PrintOnInitSet(set):
...     def __init__(self, *args, **kwargs):
...             print ("foo")
...             set.__init__(self, *args, **kwargs)
...
>>> a = PrintOnInitSet([2,7])
foo
>>> b = set([1,7])
>>> a ^ b
PrintOnInitSet([1, 2])
>>> a | b
PrintOnInitSet([1, 2, 7])
>>> a & b
PrintOnInitSet([7])
>>> a - b
PrintOnInitSet([2])
>>> a.__or__(b)
PrintOnInitSet([1, 2, 7])
>>> a.__and__(b)
PrintOnInitSet([7])
>>> a.__sub__(b)
PrintOnInitSet([2])
>>> a.__xor__(b)
PrintOnInitSet([1, 2])
>>>