Folder contains subfolders, modify subfolders image storage format (Python implementation)

Folder contains subfolders, modify subfolders image storage format, the image to png jpg image, Python code is as follows:

import os
import cv2

filePath = 'C:\\Users\\admin\\Desktop\\img'
for dirpath, dirnames, filenames in os.walk(filePath):
    path = [os.path.join(dirpath, names) for names in filenames]
    for str in path:
        if str[-3:] == 'png':
            tmp = cv2.imread(str)
            cv2.imwrite(str[:-3] + 'jpg', tmp)
            os.remove(str)