Need 'pip install pillow'

png2jpg.py

import os
from PIL import Image

def main():
    input_path = './a'
    output_path = './b'
    quality = 30

    if not os.path.isdir(output_path):
        os.mkdir(output_path)

    for filename in os.listdir(input_path):
        base, ext = os.path.splitext(filename)
        if ext == '.png':
            save_path = os.path.join(output_path, base + '.jpg')
            input_image = Image.open(os.path.join(input_path, filename))
            input_image.convert('RGB').save(save_path, quality=quality)

if __name__ == '__main__':
    main()