Saving png figures matplotlib 3.3.0 results in "AttributeError: __delete__"

Describe the issue

When trying to save png figures with ipympl, interactive widgets, and the latest matplotlib (3.3.0), the saving fails with the following error message:

import matplotlib
matplotlib.use('module://ipympl.backend_nbagg')

from matplotlib import pyplot as plt
fig = plt.figure()
fig.savefig('test.png')

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-c4b3ac84131c> in <module>
      6 from matplotlib import pyplot as plt
      7 fig = plt.figure()
----> 8 fig.savefig('test.png')

/usr/local/lib/python3.8/dist-packages/matplotlib/figure.py in savefig(self, fname, transparent, **kwargs)
   2309                 patch.set_edgecolor('none')
   2310 
-> 2311         self.canvas.print_figure(fname, **kwargs)
   2312 
   2313         if transparent:

/usr/local/lib/python3.8/dist-packages/matplotlib/backend_bases.py in print_figure(self, filename, dpi, facecolor, edgecolor, orientation, format, bbox_inches, pad_inches, bbox_extra_artists, backend, **kwargs)
   2223                 self.figure.set_edgecolor(origedgecolor)
   2224                 self.figure.set_canvas(self)
-> 2225             return result
   2226 
   2227     @classmethod

/usr/lib/python3.8/contextlib.py in __exit__(self, type, value, traceback)
    118         if type is None:
    119             try:
--> 120                 next(self.gen)
    121             except StopIteration:
    122                 return False

/usr/local/lib/python3.8/dist-packages/matplotlib/cbook/__init__.py in _setattr_cm(obj, **kwargs)
   2064         for attr, orig in origs.items():
   2065             if orig is sentinel:
-> 2066                 delattr(obj, attr)
   2067             else:
   2068                 setattr(obj, attr, orig)

AttributeError: __delete__

Saving figures as pdf or pngfigures with an older Matplotlib version (3.2.0) works well.

Versions

Matplotlib version: 3.3.0

python -c "import sys; print('\n',sys.version); import ipympl; print('ipympl version:', ipympl.__version__)" && jupyter --version && jupyter nbextension list && jupyter labextension list

–>

3.8.5 (default, Jul 20 2020, 18:32:44)
[GCC 9.3.0]
ipympl version: 0.5.7
jupyter core     : 4.6.3
jupyter-notebook : 6.0.3
qtconsole        : 4.7.4
ipython          : 7.16.1
ipykernel        : 5.3.4
jupyter client   : 6.1.6
jupyter lab      : 2.2.2
nbconvert        : 5.6.1
ipywidgets       : 7.5.1
nbformat         : 5.0.7
traitlets        : 4.3.3
Known nbextensions:
  config dir: /usr/etc/jupyter/nbconfig
    notebook section
      jupyter-js-widgets/extension disabled
  config dir: /usr/local/etc/jupyter/nbconfig
    notebook section
      bqplot/extension  enabled
      - Validating: OK
      itkwidgets/extension  enabled
      - Validating: OK
      jupyter-datawidgets/extension  enabled
      - Validating: OK
      jupyter-leaflet/extension  enabled
      - Validating: OK
      jupyter-matplotlib/extension  enabled
      - Validating: OK
      jupyter-js-widgets/extension  enabled
      - Validating: OK
  config dir: /etc/jupyter/nbconfig
    notebook section
      jupyter-js-widgets/extension  enabled
      - Validating: OK
JupyterLab v2.2.2
Known labextensions:
   app dir: /usr/local/share/jupyter/lab
        @jupyter-widgets/jupyterlab-manager v2.0.0  enabled  OK
        @jupyterlab/hub-extension v2.0.2  enabled  OK
        jupyter-matplotlib v0.7.3  enabled  OK
        jupyterlab-datawidgets v6.3.0  enabled  OK

[scimax]

For me it was fixed with the new release 0.5.8 after the fix from @ianhi . But I have to admit I moved on to the next matplotlib release already. Are you using ipympl 0.5.8?

Edit: I just saw on the readme page that ipympl 0.5.8 requires matplotlib 3.3.1. Couldn’t you move to this one? Or is 3.3.0 necessary?

[tacaswell]

and if this fixes the problem, then I would say this is the optimal solution as it does not involve core Matplotlib picking up any more complexity and reduces the exposed public surface on the ipympl side.