If you, like me, have got this error running jupyter notebooks in Ubuntu 18.04:
ModuleNotFoundError: No module named 'prompt_toolkit.formatted_text'
pip install "ipython==6.5.0"
It’s because a new release of iPython was released one month ago and it needs a newest version of prompt_toolkit module while Jupyter needs an older one. So just install the previous version of iPython (6.5.0) and everything will works fine.