matplotlib and Qt: mouse scroll event key is always None

Question

This has been fixed for now!

I am trying to make a GUI using PyQt4 with an embedded matplotlib canvas. When I scroll with the cursor over the canvas I would like to be able to control the behaviour based on an additional key press (control in this case). However, the key attribute of the mouseEvent linked to the 'scroll_event' is always None. I tested that my code correctly registers the key for a mouseEvent generated by 'button_press_event'. In the example below the on_press method correctly prints the key that was pressed at the same time. Whereas on_scoll always prints None.

How can I get access the key that was pressed during the mouse scroll event?

Thanks in advance!

import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

import matplotlib
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt4agg import NavigationToolbar2QT as NavigationToolbar
from matplotlib.figure import Figure


class GraphicTool(QMainWindow):
    def __init__(self, parent=None):
        QMainWindow.__init__(self, parent)
        self.create_menu()


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

    def on_scroll(self, event):
        print event.key
        if event.key == 'ctrl':
            # do something
            pass
        else:
            # do something else
            pass

    def create_main_frame(self):
        self.main_frame = QWidget()

        # Create the mpl Figure and FigCanvas objects.
        # 10x8 inches, 100 dots-per-inch
        self.dpi = 100
        self.fig = Figure((10.0, 8.0), dpi=self.dpi)
        self.canvas = FigureCanvas(self.fig)
        self.canvas.setParent(self.main_frame)
        self.canvas.setFocusPolicy(Qt.ClickFocus)
        self.canvas.setFocus()

        self.axes = self.fig.add_subplot(111)
        self.canvas.mpl_connect('scroll_event', self.on_scroll)
        self.canvas.mpl_connect('button_press_event', self.on_press)

        # Create the navigation toolbar, tied to the canvas
        self.mpl_toolbar = NavigationToolbar(self.canvas,
                                             self.main_frame)

        vbox = QVBoxLayout()
        vbox.addWidget(self.canvas)
        vbox.addWidget(self.mpl_toolbar)

        self.main_frame.setLayout(vbox)
        self.setCentralWidget(self.main_frame)


def main():
    app = QApplication(sys.argv)
    viewer = GraphicTool()
    viewer.show()
    viewer.raise_()
    app.exec_()

if __name__ == "__main__":
    main()

The short example above actually does work as expected. However, when I incorporate it into a bigger project it fails. I will keep debugging to see if any other events are disturbing the scroll event.

Binit Amin · Accepted Answer · 2018-02-20 13:06:28Z

1

I have checked your code. This is working as expected.

def on_scroll(self, event):
    print event.xdata
    print event.ydata
    print event.key
    if event.key == 'ctrl':
        # do something
        pass
    else:
        # do something else
        pass

This is the output of your code. I have just added xdata and ydata to your code.

0.480977867785
0.57896567718
control
0.480977867785
0.57896567718
shift
0.480977867785
0.57896567718
shift

answered Feb 20, 2018 at 13:06

Binit Amin

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1 Comment

JK Krogager Over a year ago

Mine works now as well. Don't know what has happened in between. Sorry about that!

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