如下所示：

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def draw_circle(event,x,y,flags,param):
  global ix,iy,drawing,mode,start_x,start_y

  if event == cv2.EVENT_LBUTTONDOWN:
    if drawing == False:
      start_x, start_y = x,y
      ix,iy = x,y
      drawing = True
    elif drawing == True:
      cv2.line(img,(ix,iy),(x,y),(0,255,0),3)
      ix, iy = x, y
    print(drawing)
  elif event == cv2.EVENT_MBUTTONDOWN:
    drawing = False

    cv2.line(img, (ix, iy), (start_x, start_y), (0, 255, 0), 3)
    print(drawing)


  #
  # elif event == cv2.EVENT_RBUTTONUP:
  #   cv2.line(img,(ix,iy),(x,y),(0,255,0),3)
# Next we have to bind this mouse callback function to OpenCV # # window. In the main loop, we should set a keyboard binding for
# key ‘m' to toggle between rectangle and circle.
img = np.zeros((512,512,3), np.uint8)
cv2.namedWindow('image')
cv2.setMouseCallback('image',draw_circle)

while(1):
  cv2.imshow('image',img)
  k = cv2.waitKey(1) & 0xFF
  if k == ord('m'): # 切换模式
    mode = not mode
  elif k == 27:
    break

cv2.destroyAllWindows()

本文题目：python_opencv用线段画封闭矩形的实例-创新互联
标题来源：http://pwwzsj.com/article/cdjege.html