check transparency

#! /usr/bin/python
import sys
import Image

def corners(image, test_square_size = 1):
    """returns an array of the corner bounds of the image"""
    size_x, size_y = image.size
    max_x, max_y = (size_x-1, size_y-1)
    if max_x < test_square_size or max_y < test_square_size:
        raise Exception, 'test_square_size cannot be larger than the image size'
    
    top_left        = (0,                       0,                      test_square_size,   test_square_size)
    top_right       = (max_x-test_square_size,  0,                      max_x,              test_square_size)
    bottom_left     = (0,                       max_y-test_square_size, test_square_size,   max_y)
    bottom_right    = (max_x-test_square_size,  max_y-test_square_size, max_x,              max_y)
    
    return [top_left, top_right, bottom_left, bottom_right]


def is_transparent(area):
    """returns True if the entire area is transparent, False otherwise"""
    transparents = [(255, 255, 255, 0), (0, 0, 0, 0), 255]
    max_x, max_y = area.size
    
    for x in range(0, max_x):
        for y in range(0, max_y):
            pixel = area.getpixel((x, y))
            if not pixel in transparents:
                return False    
    return True    

# Will print "Transparent"" if any of the corners are transparent
# If all the corners are non-transparent then it will print "Not transparent"
filename = sys.argv[1]

image = Image.open(filename)
for corner in corners(image):
    area = image.crop(corner)
    if is_transparent(area):
        print "Transparent"
        sys.exit(0)
print "Not transparent"