matplotlib и Qt: mouse press event.key винаги е None

Вградих фигура на matplotlib в Qt (PySide) приложение и бих искал да отговарям на събития с щракване на мишката в комбинация с модифициращи клавиши (напр. shift и control). В кода по-долу използвах метода canvas mpl_connect, за да свържа 'button_press_event' с фиктивен манипулатор, който отпечатва стойността на event.key. Тази стойност винаги е „Няма“, дори ако натисна модифициращ клавиш по време на щракване с мишката. Как мога да получа състоянието на модифициращите ключове в манипулатор на събития на мишката matplotlib, когато вграждам фигура на matplotlib в Qt приложение?

Благодаря.

import sys

from PySide import QtGui, QtCore

from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure

class MplCanvas(FigureCanvas):
    """Class to represent the FigureCanvas widget"""
    def __init__(self):
        self.fig = Figure()        
        self.ax = self.fig.add_subplot(111)

        FigureCanvas.__init__(self, self.fig)
        FigureCanvas.setSizePolicy(self, QtGui.QSizePolicy.Expanding, QtGui.QSizePolicy.Expanding)
        FigureCanvas.updateGeometry(self)

        self.fig.canvas.mpl_connect('button_press_event',self._on_press)

    def _on_press(self,event):
        print( event.key )


class MplWidget(QtGui.QWidget):
    """Widget defined in Qt Designer"""
    def __init__(self, parent = None):
        QtGui.QWidget.__init__(self, parent)

        self.canvas = MplCanvas()

        self.vbl = QtGui.QVBoxLayout()
        self.vbl.addWidget(self.canvas)

        self.setLayout(self.vbl)


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtGui.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.verticalLayout = QtGui.QVBoxLayout(self.centralwidget)
        self.verticalLayout.setObjectName("verticalLayout")
        self.mpl = MplWidget(self.centralwidget)
        sizePolicy = QtGui.QSizePolicy(QtGui.QSizePolicy.Expanding, QtGui.QSizePolicy.Expanding)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(self.mpl.sizePolicy().hasHeightForWidth())
        self.mpl.setSizePolicy(sizePolicy)
        self.mpl.setObjectName("mpl")
        self.verticalLayout.addWidget(self.mpl)
        MainWindow.setCentralWidget(self.centralwidget)
        self.statusbar = QtGui.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle(QtGui.QApplication.translate("MainWindow", "MainWindow", None, QtGui.QApplication.UnicodeUTF8))

class DesignerMainWindow(QtGui.QMainWindow, Ui_MainWindow):
    """Customization for Qt Designer created window"""
    def __init__(self, parent = None):
        super(DesignerMainWindow, self).__init__(parent)
        # setup the GUI --> function generated by pyuic4
        self.setupUi(self)


if __name__ == '__main__':

    # create the GUI application
    app = QtGui.QApplication(sys.argv)
    # instantiate the main window
    dmw = DesignerMainWindow()
    # show it
    dmw.show()
    # start the Qt main loop execution, exiting from this script
    # with the same return code of Qt application
    sys.exit(app.exec_())