Matplotlib event handling: when do they send matplotlib events and when Qt events

Question

I am trying to embed matplotlib in a Qt interface using Pyside2. I want to use 'imshow' to display an image that I can zoom into using the scrollwheel and click on as well. To connect the scrollwheel and mouse, I am using:

self.fig.canvas.mpl_connect('button_press_event', self.mouseClick)
self.fig.canvas.mpl_connect('scroll_event', self.wheelEvent)

If for the event handlers I use:

def wheelEvent(self, event):
    print(event)

def mouseClick(self, event):
    print(event)

When I run this, and then scroll and click, the output I get is:

button_press_event: xy=(94, 199) xydata=(99.99197230814553, 134.93602887023127) button=1 dblclick=False inaxes=AxesSubplot(0.125,0.290018;0.775x0.409964)
<PySide2.QtGui.QWheelEvent object at 0x7f92e3359a50>

So my question is, why in the one case am I getting a PySide event and in the other a matplotlib button_press_event? What I was expecting was to get a scroll_event.

Many thanks!

eyllanesc · Accepted Answer · 2020-01-02 04:27:59Z

2

Explanation:

The wheelEvent is a method of the QWidget class that is notified every time it interacts with the wheel when the window has the focus through a QWheelEvent and therefore the classes that inherit from it will also have it, in this case they override that method and you are also connecting it to the scroll_event event so print the QWheelEvent (in my case with PySide2 5.14 on Linux I receive QWheelEvent and scroll_event alternately).

Solution:

Do not use the wheelEvent method but another method with another name:

self.fig.canvas.mpl_connect('button_press_event', self.mouseClick)
self.fig.canvas.mpl_connect('scroll_event', self.mouseWheel)

def mouseWheel(self, event):
    print(event)

def mouseClick(self, event):
    print(event)

answered Jan 2, 2020 at 4:27

eyllanesc

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1 Comment

Mr Squid Over a year ago

Thanks for that! That would have taken >1e6 years for me to figure out...

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