def button_press_event(self, x, y, button, guiEvent=None):
self._button = button<EOL>s = '<STR_LIT>'<EOL>mouseevent = MouseEvent(s, self, x, y, button, self._key, guiEvent=guiEvent)<EOL>self.callbacks.process(s, mouseevent)<EOL>
Backend derived classes should call this function on any mouse
button press.  x,y are the canvas coords: 0,0 is lower, left.
button and key are as defined in :class:`MouseEvent`.

This method will be call all functions connected to the
'button_press_event' with a :class:`MouseEvent` instance.