$ ipython3
Python 3.4.3+ (default, Oct 10 2015, 09:15:38)
Type "copyright", "credits" or "license" for more information.

IPython 2.3.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: import matplotlib

In [2]: matplotlib.__version__
Out[2]: '1.5.0rc2'

In [3]: matplotlib.use("Qt4Agg")

In [4]: matplotlib.rcParams["backend.qt4"] = "PySide"

In [5]: from matplotlib.backends.backend_qt4agg import FigureCanvas

In [6]: from matplotlib.figure import Figure

In [7]: fig = Figure()

In [8]: canvas = FigureCanvas(fig)
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-8-9faf89066fc2> in <module>()
----> 1 canvas = FigureCanvas(fig)

/usr/lib/python3/dist-packages/matplotlib/backends/backend_qt4agg.py in __init__(self, figure)
     70             print('FigureCanvasQtAgg: ', figure)
     71         FigureCanvasQT.__init__(self, figure)
---> 72         FigureCanvasQTAggBase.__init__(self, figure)
     73         FigureCanvasAgg.__init__(self, figure)
     74         self._drawRect = None

/usr/lib/python3/dist-packages/matplotlib/backends/backend_qt5agg.py in __init__(self, figure)
     63
     64     def __init__(self, figure):
---> 65         super(FigureCanvasQTAggBase, self).__init__(figure=figure)
     66         self._agg_draw_pending = False
     67

/usr/lib/python3/dist-packages/matplotlib/backends/backend_qt4.py in __init__(self, figure)
     69
     70         # Note different super-calling style to backend_qt5
---> 71         QtWidgets.QWidget.__init__(self)
     72         FigureCanvasBase.__init__(self, figure)
     73         self.figure = figure

RuntimeError: You can't initialize an object twice!

In [9]: