class Foo: a = traits.HasCallback()
class Foo: a = traits.HasValidator() + traits.HasCallback()
class Foo: a = traits.HasCallback([callback_1, callback_2])
class Foo: a = traits.HasCallback()
class Foo: a = traits.HasValidator() + traits.HasCallback()
class Foo: a = traits.HasCallback([callback_1, callback_2])