ix=0
iy=0
cropping=False
refPt=[]

def draw(event, x, y, flags, param):
    global ix, iy, drawing, right_mouse_button, left_mouse_button, \
        refPt, cropping, color, thickness
    ix = int(x)
    iy = int(y)

    if event == cv2.EVENT_MBUTTONDOWN:
        if not cropping:
            refPt = [(x, y)]
            cropping = True
        else:
            cropping = False
            refPt.append((x, y))
            cv2.rectangle(o2, refPt[0], refPt[1], color, thickness, cv2.LINE_8)

cv2.setMouseCallback('normal', draw)

o2 = cv2.rectangle (o2, (refPt[0]), (refPt[1]), (0, 255, 0), 3)