[Opencv] draw event

import numpy as np
import cv2
from math import sqrt, pow


img = np.zeros((512, 512, 3), np.uint8)
img_ = img.copy()
drawing = False
mode = True
(ix, iy) = (-1, -1)

windowName = 'Drawing'
cv2.namedWindow(windowName)

green = (0, 255, 0)
red = (0, 0, 255)
 
def draw(event, x, y):
    global ix, iy, drawing, mode, img, img_
    
    if event == cv2.EVENT_LBUTTONDOWN:
        drawing = True
        (ix, iy) = x, y
    
    elif event == cv2.EVENT_MOUSEMOVE:
        if drawing == True:
            if mode == True:
                img = img_.copy()
                cv2.rectangle(img, (ix, iy), (x, y), green, 2)

    elif event == cv2.EVENT_LBUTTONUP:
        drawing = False
        if mode == True:
            cv2.rectangle(img, (ix, iy), (x, y), green, 2)
            img_ = img.copy()
 
cv2.setMouseCallback(windowName, draw)
        
def main():
    global mode
    
    while(True):
        cv2.imshow(windowName, img)
        
        key = cv2.waitKey(1)
        if key == ord('m') or key == ord('M'):
            mode = not mode 
        elif key == 27:
            break
            
    cv2.destroyAllWindows()
 
if __name__ == '__main__':
    main()