Segmentation fault on Qt5Agg when using the wrong linestyle

[Dapid]

The following program triggers an Python error and a segmentation fault (notice that the correct linestyle is -.):

import matplotlib
matplotlib.use('Qt5Agg')
import matplotlib.pyplot as plt
plt.hist([0, 1, 1, 2], linestyle='.-')
plt.show()

plot sanitizes the input, so this just produces a ValueError.
import matplotlib.pyplot as plt
plt.plot([0, 1], [1, 2], linestyle='.-')

The backends Qt4Agg and TkAgg also raise an error, but don't produce a hard crash.

My system is Python 3.5 on a virtual environment with matplotlib built from source via pip.

Full traceback for the aborting snippet:

Traceback (most recent call last):
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/backends/backend_qt5agg.py", line 176, in __draw_idle_agg
    FigureCanvasAgg.draw(self)
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/backends/backend_agg.py", line 474, in draw
    self.figure.draw(self.renderer)
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/artist.py", line 61, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/figure.py", line 1159, in draw
    func(*args)
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/artist.py", line 61, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/axes/_base.py", line 2324, in draw
    a.draw(renderer)
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/artist.py", line 61, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/patches.py", line 494, in draw
    gc.set_linestyle(self._linestyle)
  File "/home/david/.virtualenv/py35/lib/python3.5/site-packages/matplotlib/backend_bases.py", line 1064, in set_linestyle
    raise ValueError('Unrecognized linestyle: %s' % str(style))
ValueError: Unrecognized linestyle: .-
Aborted (core dumped)

[tacaswell]

What version of mpl are you using?

[Dapid]

Sorry, I forgot. 1.5.1, latest on Pypi

[anntzer]

I can confirm the issue as of 1.5.1 and PyQt 5.6 (5.7 doesn't work with 1.5.1), but it is fixed as of 2.0b3 with both PyQt5.6 and 5.7.

[tacaswell]

@anntzer do we have any idea what fixed it? There is likely to still be a 1.5.3.

[anntzer]

Bisected back to 3ef3f5b (basically, throwing exceptions at draw time is always going to be fatal for Qt5).

[tacaswell]

ew, that is not something we can backport to 1.5

I suspect we are going to be playing wack-a-mole with qt5 for a while if raising at draw time is fatal...

[anntzer]

See http://pyqt.sourceforge.net/Docs/PyQt5/incompatibilities.html#unhandled-python-exceptions
Basically you can avoid this by overwriting sys.excepthook, but I'd rather not go that route...

[tacaswell]

I guess in __draw_idle_agg we could catch and print the exceptions? That would get us back to the status-quo without overriding it for the rest of qt5?

[anntzer]

I guess that could work, yes.