Sorts images by a number of non-white pixels

sort_images.py

import cv, cv2
import numpy as np
import sys

def read(filenames):
    files = []
    for filename in filenames:
        img = cv2.imread(filename)
        gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
        files.append(gray)
    return files

def sort_by_black_count(files):
    return sorted(files, key=lambda img: cv2.countNonZero(img))

def write(files):
    for i,img in enumerate(files):
        cv2.imwrite('sorted/%d.png' % i, img)

def sort(filenames):
    files = read(filenames)
    files = sort_by_black_count(files)
    write(files)


if __name__ == '__main__':
    filenames = sys.argv[1:]
    sort(filenames)