Impulse salt and pepper image noise

import cv2
import numpy as np
import random
 
if  __name__ == "__main__":
 
    img = cv2.imread("Lenna.jpg")
 
    prob_noise = 0.01
    thres = 1 - prob_noise
 
    for i in range(0, img.shape[0]):
        for j in range(0, img.shape[1]):
 
            rdn = random.random()
 
            if rdn < prob_noise:
                img[i][j] = 0
            elif rdn > thres:
                img[i][j] = 255
            else:
                img[i][j] = img[i][j]
 
    cv2.imwrite("Lenna_salt_pepper_noise.jpg", np.uint8(img))