class ValidatorMeta(type):
    def __new__(mcs, name, bases, attrs):
        assert 'x' not in attrs, "Using properties isn't allowed"
        assert '__setattr__' not in attrs, "Using __setattr__ isn't allowed"
        return super().__new__(mcs, name, bases, attrs)
 
class SomeClass(metaclass=ValidatorMeta):
    @property
    def __dict__(self):
        return {'y': 5}
 
obj = SomeClass()
obj.x = 3
obj.y = 5
assert vars(obj) == {'y': 5}
print('You win!')

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