Check if a file is valid PNG

pngchecker.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import os
import argparse

from PIL import Image

# Get filename from arguments
parser = argparse.ArgumentParser(description='get arguments')
parser.add_argument('filename')

args = parser.parse_args()
print args.filename

# Check if file exists
if not os.path.isfile(args.filename):
    raise IOError('Is not a file')

# TODO: Check if file is valid png
im = Image.open(args.filename)
im.verify()

You can do this without requiring PIL. Simply get the file contents and match the first chars ([:8]) with "\x89PNG\r\n\x1a\n" and char [12:16] with "IHDR".