universalset.py

class UniversalSet(set):
    def __init__(self):
        super(UniversalSet, self).__init__([])

    def __and__(self, other):
        if not isinstance(other, set):
            other = set(other)
        return other

    def __rand__(self, other):
        if not isinstance(other, set):
            other = set(other)
        return set(other)

    def __or__(self, other):
        return self

    def __ror__(self, other):
        return self

    def __contains__(self, other):
        return True

    def __str__(self):
        return '{all}'

    def intersection(self, other):
        return self.__and__(other)

    def union(self, other):
        return self.__or__(other)


uset = UniversalSet()


def test_uset_membership():
    assert 4 in uset
    assert 'a' in uset
    assert float('nan') in uset
    assert None in uset
    assert uset in uset

def test_uset_intersection():
    assert uset & {1, 3, 5} == {1, 3, 5}
    assert {1, 2, 3} & uset == {1, 2, 3}
    assert uset & uset == uset
    assert uset.intersection({0, 2, 4}) == {0, 2, 4}

def test_reverse_intersection_FAILS():
    assert {0, 2, 4}.intersection(uset) == {0, 2, 4}

def test_uset_union():
    assert uset | {1, 3, 5} == uset
    assert {1, 3, 5} | uset == uset
    assert uset | set() == uset
    assert uset | uset == uset
    assert uset.union({0, 2, 4}) == uset

def test_reverse_union_FAILS():
    assert {0, 2, 4}.union(uset) == uset