def METHOD_NAME(self):
    class BaseFoo(HasTraits):
        # The default value set in the class definition is "0"
        bar = Expression()
    class Foo(BaseFoo):
        default_calls = Int(0)
        def _bar_default(self):
            self.default_calls += 1
            return "3"
    f = Foo()
    self.assertEqual(f.bar, "3")
    self.assertEqual(eval(f.bar_), 3)
    self.assertEqual(f.default_calls, 1)