Answer by meduz for ipython server can't launch: No module named notebook.notebookapp

I received the same problem when upgrading IPython. At the moment the answer was written, it was a bug linked to the latest 4 version. If a similar problem occurs for which you wish to switch back to the stable version 3.2.1:

pip uninstall -y IPythonpip install ipython==3.2.1

• note: the -y option indicates "yes I want to uninstall" with no interaction.
• note 2: possible duplicate in ImportError: No module named notebook.notebookapp