FigureCanvas.motion_notify_event and other event handlers have different signatures across backends:

lib/matplotlib/backend_bases.py
1893:    def motion_notify_event(self, x, y, guiEvent=None):

lib/matplotlib/backends/backend_gtk.py
321:    def motion_notify_event(self, widget, event):

lib/matplotlib/backends/backend_tkagg.py
386:    def motion_notify_event(self, event):

lib/matplotlib/backends/backend_gtk3.py
263:    def motion_notify_event(self, widget, event):

Note that the default signature is written in such a way that one can programmatically trigger an event on the canvas without having to instantiate a native event object, which may be tricky (yes, you could always mock the event but that's obviously not optimal). It would be preferrable if all backends kept the same signature (which would probably involve breaking backcompat).