I'm trying to write a code in Python that should read a video and at any time the user must be able to draw a rectangular area on it. After that I use the rectangle coordinates to extract the area selected from the current frame. This smaller area is called region in my code and running this code on spyder, this variable remains empty. I don't know why, since rect is composed of 4 integer numbers. Thank you in advance for your help.

import cv2

rect = (0,0,0,0)
startPoint = False
endPoint = False
selected_ROI = False

def on_mouse(event,x,y,flags,params):

    global rect,startPoint,endPoint, selected_ROI

    if event == cv2.EVENT_LBUTTONDOWN:

        if startPoint == True and endPoint == True:
            startPoint = False
            endPoint = False
            rect = (0, 0, 0, 0)
            selected_ROI = False

        if startPoint == False:
            rect = (x, y, 0, 0)
            startPoint = True

        elif endPoint == False:
            rect = (rect[0], rect[1], x, y)
            endPoint = True
            selected_ROI = True

video = cv2.VideoCapture('giri_motore.mkv')

while video.isOpened():
    # Read frame

    ret, frame = video.read()

    cv2.namedWindow('original')
    cv2.setMouseCallback('original', on_mouse)

    if selected_ROI:
        cv2.rectangle(frame, (rect[0], rect[1]), (rect[2], rect[3]), (255, 0, 255), 2)

  
    region = frame[rect[0]:rect[1], rect[2]:rect[3]]
    cv2.imshow('original', frame)
    

    key = cv2.waitKey(1)
    if key == ord('q'):
        break

video.release()
cv2.destroyAllWindows()