def METHOD_NAME(self):
    class TestClass(HasTraits):
        atr = Union(Int(3), Float(4.1), Str("Something"))
    self.assertEqual(TestClass().atr, 3)
    class TestClass(HasTraits):
        atr = Union(
            Int(3), Float(4.1), Str("Something"),
            default_value="XYZ",
        )
    self.assertEqual(TestClass().atr, "XYZ")
    class TestClass(HasTraits):
        atr = Union()
    self.assertEqual(TestClass().atr, None)
    class TestClass(HasTraits):
        atr = Union(None)
    self.assertEqual(TestClass().atr, None)