Notebook backend broken on Master

[jenshnielsen]

Just trying to do something simple like:

%matplotlib notebook
import matplotlib.pyplot as plt
plt.plot(range(10))
plt.tight_layout()

results in

Traceback (most recent call last):
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/ipykernel/comm/manager.py", line 137, in comm_msg
    comm.handle_msg(msg)
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/ipykernel/comm/comm.py", line 164, in handle_msg
    self._msg_callback(msg)
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/matplotlib/backends/backend_nbagg.py", line 304, in on_message
    self.manager.handle_json(message)
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/matplotlib/backends/backend_webagg_core.py", line 485, in handle_json
    self.canvas.handle_event(content)
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/matplotlib/backends/backend_webagg_core.py", line 272, in handle_event
    return handler(event)
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/matplotlib/backends/backend_webagg_core.py", line 288, in handle_draw
    self.draw()
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/matplotlib/backends/backend_webagg_core.py", line 181, in draw
    self.manager.refresh_all()
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/matplotlib/backends/backend_webagg_core.py", line 489, in refresh_all
    diff = self.canvas.get_diff_image()
  File "/Users/jhn/Envs/matplotlibbisect/lib/python3.5/site-packages/matplotlib/backends/backend_webagg_core.py", line 232, in get_diff_image
    None, compression=6, filter=_png.PNG_FILTER_NONE)
TypeError: Object does not appear to be a 8-bit string path or a Python file-like object

git bisect tells me that it was broken by 9758da5

[mdboom]

Remilestoned to 1.5.2, since #5910 was backported to there and now has this bug as well.

[jenshnielsen]

I can't reproduce it on 1.5.x I guess the backport does stuff slightly different?

[mdboom]

Yes -- you were right, actually. Only master has the "pass None to _png.write_png to write to a string buffer" optimization that's affected by this.