Resize all images in a folder using python

resize_image.py

import os, sys
from PIL import Image

h = 256
w = 144
in_dir = "./input_dir/"
out_dir = "./output_dir/"
dirs = os.listdir( in_dir )

def resize():
    for item in dirs:
        if os.path.isfile(in_dir+item):
            im = Image.open(in_dir+item)
            f, e = os.path.splitext(in_dir+item)
            imResize = im.resize((w, h), Image.ANTIALIAS)
            print(f)
            imResize.save(out_dir + item, 'jpeg', quality=75)

resize()

why the code is not working for me? I want to resize multiple images in a folder.