While I agree with you, I will note that even set union in Python is not commutative. a | b should equal b | a in the sense of __eq__, but the actual objects in

While I agree with you, I will note that even set union in Python is not commutative. a | b should equal b | a in the sense of __eq__, but the actual objects in the result set depend on the order of the arguments (and in the opposite way from dict + dict). This happens with objects that are distinct but compare/hash equally (x is not y and x == y). Whether that actually matters for any useful program is another story... Dumb program to illustrate this point: class Dummy: def __init__(self, value): self.value = value def __repr__(self): return 'Dummy(%s)' % self.value def __hash__(self): return 0 def __eq__(self, other): return True a = {Dummy(0)} b = {Dummy(1)} print(a | b) print(b | a) print(a | b == b | a)