Seems to be the same as yesterday's #11365. Did anything else change outside of matplotlib recently such that this would suddenly come up twice within less than a day?

How are you starting the notebook server?

Starting from a bash script:

#!/usr/bin/env bash
export PYTHONSTARTUP=path/to/my_init.ipy
jupyter notebook

and the my_init.ipy file is

%matplotlib notebook

This is not likely due to a dependency as I can get rid of the problem by switching back from 2.2.2 to 2.0.2 using a pip3 uninstall and then pip3 install matplotlib==2.0.2. Everything else in my installation remains unchanged.

If you start up without your init script does it work as expected?

This is my fault. When installing 2.0.2, I have to modify backends/backend_nbagg.py by adding

def flush_events(self): pass

to the FigureCanvasNbAgg class to work around the NotImplementedError exception that is encountered in the parent class virtual method. I was rather carelessly adding the same line to the 2.2.2 version of the file, following installation, which was landing in the wrong place. This caused aa visible error message when manually typing %matplotlib notebook at the notebook prompt.

Looks like code has changed in 2.2.2 so that we don't get this exception any longer. I didn't dig in further as to why.

Glad that got sorted!