creating instance with Undefined is not meant to assign Undefined

[minrk]

In traitlets, the Undefined sentinel is meant to be a placeholder for no value having been specified. However, some traittypes tests explicitly pass Undefined as the default value, paradoxically explicitly stating that a value has not been specified.

In traitlets 4.3, this results (I think incorrectly) in the value Undefined being set as the actual value of the trait. In traitlets master, the result on accessing the trait with Undefined value as an error, saying the value is undefined.

I'm not sure what use case these tests are meant to represent. Is it important to be able to explicitly store Undefined in traits? If so, it should probably be a different sentinel than the one in traitlets intended to mean "this has no value yet, fail on attempts to access it".

[vidartf]

It has been a while since I touched this, but at least in ipydatawidgets I think it might have been "significant" in order to work around the previous missing option of sending buffers with comm open messages from JS. I.e. I needed a way of saying "this will come later, do not create a default value yet".