FigureCanvasQT backend_qt5agg bug (backend_qt4agg) works

[atsju]

In this example,
http://matplotlib.org/examples/user_interfaces/embedding_in_qt4.html (embedding_in_qt4.py)
When replacing
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
through
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg

I got this error
File "C:\Python34\lib\site-packages\matplotlib\backends\backend_qt5.py", line 238, in init
super(FigureCanvasQT, self).init(figure=figure)

When looking at class FigureCanvasQT in backend_qt5.py and backend_qt4.py it seems that super is calling init on first inherited class (QtWidgets.QWidget) instead of second one (FigureCanvasBase).

Let me know if I was not clear enough and if it is realy a bug or a missusage.

[jenshnielsen]

This sounds similar to #3623. It turned out that this was because I had the env variable QT_API set to pyqt which resulted in pyqt4 being imported into the qt5 backend. This is fixed in 1.4.1rc1

That being said the line still looks wrong to me. You are right that super will first call init on the first inherited class and if not found move on according to the method resolution order: http://stackoverflow.com/questions/1848474/method-resolution-order-mro-in-new-style-python-classes
This should not work since init of QtWidgets.QWidget doesn't take a figure keyword argument.

But any combination of explicitly calling init on the two parent classes that I have tried fails.

[atsju]

Could you be more specific ? What should I do
-Use matplotlib 1.4.1rc1 (download from where and how to install SIMPLY under windows ?)
OR
-Set QT_API to someting other (how and to what ?)

[jenshnielsen]

The easies is to check if the environmental variable is set.
You should be able to check the variable from python with something like:

import os
print os.environ.get('QT_API')

If this prints either pyqt or pyside it is likely this issue that you are seeing. Otherwise something else is wrong.

Without knowing how you have installed python and matplotlib it is hard to tell you how to install matplotlib but usually nothing is easy or simple on windows... If you have installed all the packages from scratch you might be able to use the one from here http://www.lfd.uci.edu/~gohlke/pythonlibs/#matplotlib
build by Christoph Gohlke

[atsju]

QT_API isn't set.
I'm using QT5.3.2 python 3.4.1 PyQT4
Used mathplotlib1.4.0
Uninstalled 1.4.0 and installed 1.4.1.rc1
Still the same error.

When I import FigureCanvasQTAgg from backend_qt4agg it is working well even if I have only QT5 installed and not QT4.

[jenshnielsen]

Thanks for testing this confirms that there is some deeper problem somewere. I will try to have a closer look of what is happening but it might take some time

[atsju]

I workaround using qt4agg. Hope I was helpfull.
Thank you for your ultraquick responses