Closing a figure from the notebook does not close the python figure

[jenshnielsen]

This is essentially a regression of matplotlib/matplotlib#4841 which happened when the nbagg backend was converted to a widget.

To reproduce create a cell with a simple plot.

plt.plot(range(10))
plt.show()

re execute the cell and observe that the figure count goes up and memory consumption goes up too. If you continue doing this you will eventually have more than 20 open figures and matplotlib will print a warning even if you only have one figure displayed.

The same is true if you explicitly close the widget using the widget close cross on the left of the figure.

I have spent some time trying to figure out how to best fix this but don't really know how to best do this. This function
is meant to trigger a close message to the python side when the figure is removed from the DOM but no longer works.

The old no longer existing close button is also intended to send a close signal to the python layer but this is not hooked up to the widget close button.

matplotlib/matplotlib#6414 has some more related issues.

[jenshnielsen]

@SylvainCorlay Do you have any idea of how to hook this up to the widgets and send a close message to python when the widget is destroyed/closed from the notebook

[SylvainCorlay]

@jenshnielsen we can't to the latter because the close button only destroys a view. There may be other views of the same widget. And even when there is no view, the widget model could be wired to other widget models in the front end. Essentially it is a cross-language reference counting problem.

[jenshnielsen]

That makes sense.

There is is certain mismatch between the Matplotlib APIs which and designed around GUI frameworks where there is a 1 to 1 relation ship between a figure window and it's python representation and the widgets. We will have to think more about how to handle this

[SylvainCorlay]

We could close the figure when the jupyter comm is closed, and reversely. That would be a sensible thing to do in my opinion.

[jasongrout]

As @jkrumbiegel points out at jupyterlab/jupyterlab#5373, it can also be confusing to get this error by just evaluating the cell creating the figure over and over again.

[gsaurabhr]

Any update on this? This issue still persists...

[SylvainCorlay]

@gsaurabhr as per comment above, this is somewhat by design:

@jenshnielsen we can't to the latter because the close button only destroys a view. There may be other views of the same widget. And even when there is no view, the widget model could be wired to other widget models in the front end. Essentially it is a cross-language reference counting problem.

This gets more complicated if you account from multiple notebooks / front-end connected to the kernels.