Face Detection(video) - Python(OpenCV)

import cv2

face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
#Read the input image
#img = cv2.imread('test.png')
cap = cv2.VideoCapture('20180423_182625.mp4') #zero for webcam

while cap.isOpened():
    _, img = cap.read()

    gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
    faces = face_cascade.detectMultiScale(gray, 1.1, 4)
    for(x, y, w, h) in faces:
        cv2.rectangle(img, (x,y), (x+w, y+h), (255, 0, 0), 3)
    #Display the output
    cv2.imshow('img', img)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
cap.release()