gistfile1.py

class ConnectionButton(MTWidget):
   instances = []

    def __init__(self, **kwargs):
       super().__init__(**kwargs)
       ConnectionButton.instances.append(self)

    def on_touch_down(self, touch):
       if not self.collide_point(*touch.pos):
         return
       self.connect_touch(touch)

    def connect_touch(self, touch):
       def touch_up(touch):
         for target in ConnectionButton.instances:
           if target.collide_point(*touch.pos):
             self.connect(target)
       def touch_draw(touch):
         drawLine([touch.userdata['connection_start'], touch.pos])

       touch.userdata['connection_start'] = self.to_window(*self.pos)
       touch.grab(self, up=touch_up)
       self.connect('on_draw', curry(touch_draw, touch))