@minrk
Regarding the places where I had to add allow_none=True. it is interesting to check whether it was expected, specifically regarding the enums in

shellapp.py
interactiveshell.py
test_message_spec.py

Good point. I bet it's not all of them.

Thanks for this!

Done, I have replaced the allow_none=True with default values. The chosen default value is the first value proposed in the Enum. Travis is happy.

When #7770 is merged, it will become sensible to have allow_none=False for Instance and Type as well, since unused default values will not be instantiated.

Then, it would be nice to not have any allow_none=True by default in all trait types, but I will make it a different PR because there are much more cases in IPython where it will be required to forcibly set allow_none=True for Type or Instance trait type.

Removed some redundant allow_none=False now that it is the default.

I think this might be causing some failures in IPython.parallel: "The 'stop_data' trait of a LocalEngineSetLauncher instance must be a dict, but a value of class 'NoneType' (i.e. None) was specified."

Thanks, I'll make sure the parallel tests pass and open a PR with the few remaining changes. This was one part, but there's another as well.

Sorry I missed this one. On this regard, after the default value validation PR is merged, I would like to do the same with Type and Instance (have allow_none=False by default).
I will make sure to run the parallel tests.