NotebookApp.launch_instance() fails when _ is defined

[SimonBiggs]

Open up a python interactive prompt by typing 'python' within a terminal.

Copy and run the following lines of code into the python prompt:

from notebook.notebookapp import NotebookApp
'a'
NotebookApp.launch_instance()

The following error is raised:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/simon/.pyenv/versions/py36/lib/python3.6/site-packages/jupyter_core/application.py", line 266, in launch_instance
    return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
  File "/home/simon/.pyenv/versions/py36/lib/python3.6/site-packages/traitlets/config/application.py", line 658, in launch_instance
    app.start()
  File "/home/simon/.pyenv/versions/py36/lib/python3.6/site-packages/notebook/notebookapp.py", line 1584, in start
    for line in self.notebook_info().split("\n"):
  File "/home/simon/.pyenv/versions/py36/lib/python3.6/site-packages/notebook/notebookapp.py", line 1527, in notebook_info
    info = self.contents_manager.info_string() + "\n"
  File "/home/simon/.pyenv/versions/py36/lib/python3.6/site-packages/notebook/services/contents/filemanager.py", line 543, in info_string
    return _("Serving notebooks from local directory: %s") % self.root_dir
TypeError: 'str' object is not callable

It appears to be due to the global variable _ being used here:

https://github.com/jupyter/notebook/blob/master/notebook/services/contents/filemanager.py#L543