def METHOD_NAME(self):
    element = ElementBase()
    self.assertIsNone(element.default_value)
    element.fixed = "foo"
    self.assertEqual("foo", element.default_value)
    element.default = "bar"
    self.assertEqual("bar", element.default_value)
    element.default = ""
    self.assertEqual("", element.default_value)
    element.default = None
    element.fixed = ""
    self.assertEqual("", element.default_value)