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Draw elastic shapes on a canvas on drag, move on right click

from Tkinter  import * trace = 0        class CanvasEventsDemo:      def __init__(self, parent=None):         canvas = Canvas(width=300, height=300, bg='white')          canvas.pack()         canvas.bind('<ButtonPress-1>', self.onStart)           canvas.bind('<B1-Motion>',     self.onGrow)            canvas.bind('<Double-1>',      self.onClear)           canvas.bind('<ButtonPress-3>', self.onMove)            self.canvas = canvas         self.drawn  = None     def onStart(self, event):         self.start = event         self.drawn = None     def onGrow(self, event):                                   canvas = event.widget         if self.drawn: canvas.delete(self.drawn)         objectId = canvas.create_oval(self.start.x, self.start.y, event.x, event.y)         if trace: print objectId         self.drawn = objectId     def onClear(self, event):         event.widget.delete('all')     def onMove(self, event):         if self.drawn:                         if trace: print self.drawn             canvas = event.widget             diffX, diffY = (event.x - self.start.x), (event.y - self.start.y)             canvas.move(self.drawn, diffX, diffY)             self.start = event       CanvasEventsDemo() mainloop()