TrackBar(example1)- OpenCV, python

import numpy as np
import cv2 as cv

def nothing(x):
    print(x)

#create a black image, a window
img = np.zeros((300, 512, 3), np.uint8)
cv.namedWindow('image')

cv.createTrackbar('B', 'image', 0, 255, nothing)
cv.createTrackbar('g', 'image', 0, 255, nothing)
cv.createTrackbar('R', 'image', 0, 255, nothing)

#adding a switch(on/Off)
switch = 'O : OFF\n 1 : ON'
cv.createTrackbar(switch, 'image', 0, 1, nothing)

while(1):
    cv.imshow('image', img)
    k=cv.waitKey(1)
    if k==27:
        break
    b = cv.getTrackbarPos('B', 'image')
    g = cv.getTrackbarPos('G', 'image')
    r = cv.getTrackbarPos('R', 'image')
    s = cv.getTrackbarPos(switch, 'image')

    if s==0:
        img[:] = 0
    else:
        #change image color
        img[:] = [b, g, r]

cv.destroyAllWindows()