class Canvas:
    def draw_pic(self, shape):
        print('--start draw--')
        shape.draw(self)

class Rectangle:
    def draw(self, canvas):
        print('draw rectangle on %s' % canvas)

class Triangle:
    def draw(self, canvas):
        print('draw triangle on %s' % canvas)

class Circle:
    def draw(self, canvas):
        print('draw circle on %s' % canvas)

c = Canvas()
c.draw_pic(Rectangle())
c.draw_pic(Triangle())
c.draw_pic(Circle())

 

 

谢谢！