A numeric diff of two images

pixel_diff.py

#!/bin/python2.7
from itertools import izip
from PIL import Image
import argparse
import os

parser = argparse.ArgumentParser()
#parser.add_argument("--preserve_case", "-p", action = 'store_true')
# parser.add_argument("--file-list", "-f", action = 'store_true')
parser.add_argument("files", nargs = 2, type = str, default = None)

args = parser.parse_args()

try:
    i1 = Image.open(args.files[0])
except Exception as e:
    print "error opening LHS " + args.files[0]
    raise e
try:
    i2 = Image.open(args.files[1])
except Exception as e:
    print "error opening RHS " + args.files[1]
    raise e

assert i1.mode == i2.mode, "Different kinds of images: " + args.files[0] + " " + args.files[1]
assert i1.size == i2.size, "Different sizes: " + args.files[0] + " " + args.files[1]

pairs = izip(i1.getdata(), i2.getdata())
if len(i1.getbands()) == 1:
    # for gray-scale jpegs
    dif = sum(abs(p1 - p2) for p1, p2 in pairs)
else:
    dif = sum(abs(c1 - c2) for p1, p2 in pairs for c1, c2 in zip(p1, p2))

ncomponents = i1.size[0] * i1.size[1] * 3
print "Difference (percentage) Pixels: {:>6.1%}".format((dif / 255.0) / ncomponents),
statinfo = os.stat(args.files[0])
i1size = statinfo.st_size
statinfo = os.stat(args.files[1])
i2size = statinfo.st_size

print " Size: {:>6.1%} ".format(float(i2size) / i1size),
print args.files[0]