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Switching to log scale when there is no positive data crashes the Qt5 backend, causes inconsistent internal state in others #6852
Description
mpl 1.5.1/2.0b3/master, Qt5Agg backend, Python 3.5.2, Arch Linux
plt.plot([-1, -2]); plt.show()
and press l while the mouse is over the axes.
Qt5 aborts Python due to the exception propagating out of the slot:
/usr/lib/python3.5/site-packages/matplotlib/axes/_base.py:2270: RuntimeWarning: invalid value encountered in double_scalars
delta = (x1t - x0t) * margin
Traceback (most recent call last):
File "/usr/lib/python3.5/site-packages/matplotlib/backends/backend_qt5.py", line 314, in keyPressEvent
FigureCanvasBase.key_press_event(self, key, guiEvent=event)
File "/usr/lib/python3.5/site-packages/matplotlib/backend_bases.py", line 1842, in key_press_event
self.callbacks.process(s, event)
File "/usr/lib/python3.5/site-packages/matplotlib/cbook.py", line 550, in process
proxy(*args, **kwargs)
File "/usr/lib/python3.5/site-packages/matplotlib/cbook.py", line 417, in __call__
return mtd(*args, **kwargs)
File "/usr/lib/python3.5/site-packages/matplotlib/backend_bases.py", line 2639, in key_press
key_press_handler(event, self.canvas, self.canvas.toolbar)
File "/usr/lib/python3.5/site-packages/matplotlib/backend_bases.py", line 2546, in key_press_handler
ax.set_yscale('log')
File "/usr/lib/python3.5/site-packages/matplotlib/axes/_base.py", line 3146, in set_yscale
self.autoscale_view(scalex=False)
File "/usr/lib/python3.5/site-packages/matplotlib/axes/_base.py", line 2288, in autoscale_view
margins['bottom'], margins['top'], self.set_ybound)
File "/usr/lib/python3.5/site-packages/matplotlib/axes/_base.py", line 2278, in handle_single_axis
x0, x1 = locator.view_limits(x0, x1)
File "/usr/lib/python3.5/site-packages/matplotlib/ticker.py", line 1803, in view_limits
"Data has no positive values, and therefore can not be "
ValueError: Data has no positive values, and therefore can not be log-scaled.
Fatal Python error: Aborted
Not sure what the best approach is but I think we should certainly try to avoid a fatal crash...
While it may be tempting to just wrap key_press_handler in a try... except or disable the abort()ing behavior of PyQt5 by installing a custom excepthook (see http://pyqt.sourceforge.net/Docs/PyQt5/incompatibilities.html section for PyQt5.5), this would hide the fact that the canvas is left in an inconsistent state as the axes internally believe to have switched to log-scale. This can be seen e.g. by using the Qt4Agg backend (where the exception is printed but the program not abort()ed), by typing l and then trying to zoom in (again with the mouse): the canvas becomes completely empty in that case.
Activity
LindyBalboa commentedon Jul 29, 2016
Would it perhaps make sense to change the default to
symlog? I know it is only suppose to be used for exceptional sign changes to standard math (I use it all the time for transistor curves), but I think it would lead to a more intuitive behavior.Edit: Shouldn't this be handled by
nonposynonposxin mpl.axes.Axes.set_yscale?LindyBalboa commentedon Jul 30, 2016
I posted here last night, but maybe forgot to submit?
I played around with it and it seems
nonposyis only successfully handled viamaskingandclippingif there is at least one positive value. The existing structures don't seem too well suited to handle this situation. The way it is, I would guess the smallest effective change would be to add aall(i<0 for i in y)somewhere inside `set_x/yscale' or the appropriate method called from it.I personally would still recommend changing the default behavior of
landktosymlog. Although it isn't mathematically correct, it has more real world applications and is more intuitive from a graphical sense.anntzer commentedon Aug 26, 2016
Actually my comment about wrapping in a try... except was incorrect: that can work, one simply has to restore the original scale in case of a failure. See #6983.
tacaswell commentedon Aug 27, 2016
Closed by #6983
please re-open if I am confused.