python按照比例缩小图片并生成新的图片

import os
from PIL import Image
ext = ['jpg','jpeg','png']
files = os.listdir('.')

def process_image(filename, mwidth=200, mheight=400):
    image = Image.open(filename)
    w,h = image.size
    if w<=mwidth and h<=mheight:
        print(filename,'is OK.')
        return 
    if (1.0*w/mwidth) > (1.0*h/mheight):
        scale = 1.0*w/mwidth
        new_im = image.resize((int(w/scale), int(h/scale)), Image.ANTIALIAS)

    else:
        scale = 1.0*h/mheight
        new_im = image.resize((int(w/scale),int(h/scale)), Image.ANTIALIAS)     
    new_im.save('new-'+filename)
    new_im.close()

for file in files:
    if file.split('.')[-1] in ext:
        process_image(file)