ipython/ipython — pull request #4188 by takluyver (2013-09-09 17:17:33+00:00)
Title: Allow using None as a default value for Dict traitlets

This was converting None to an empty dict, which caused the application to pass in a user_ns to the shell, which then behaves slightly differently, creating a DummyMod instance.

Ping @minrk : Is this the best way to fix this?