#對於弱型別的語言來說, 變數沒有申明型別,因此同一個變數完全可以再不同的事件引用不同的物件,當同一個變數再呼叫同一個方法時候,完全可以呈現出多種行為
#這就是多型
#DrawCanvas的draw_canvas方法傳入一個shape引數,該方法就是呼叫shape引數的draw方法,將自己繪製再畫布上
class DrawCanvas:
    def draw_canvas(self,shape):
        shape.draw(self)

class Rectangle:
    def draw(self,canvas):
        print ("draw rectangle in %s
 
" % canvas)

class Triangle:
    def draw(self,canvas):
        print "draw triangle in %s" % canvas

class Circle:
    def draw(self,canvas):
        print "draw circle in %s" % canvas

c = DrawCanvas()
c.draw_canvas(Rectangle())
c.draw_canvas(Triangle())
c.draw_canvas(Circle())

draw rectangle in
 
 <__main__.DrawCanvas instance at 0x0000000003815B88>
draw triangle in <__main__.DrawCanvas instance at 0x0000000003815B88>
draw circle in <__main__.DrawCanvas instance at 0x0000000003815B88>