def joined_fragments image assert image mode is frags for in xy image

def joined_fragments(image):
    assert image.mode is '1'

    frags = {}

    for x, y in xy(image):
        if image.getpixel((x,y)) == 0: # black pixel
            new_fragment = set([(x, y)])
            frags[(x,y)] = new_fragment

            for dot in (
                    (x-1, y-1), (x+0, y-1), (x+1, y-1),
                    (x-1, y+0),):#             (x+1, y+0),

                if frags.has_key(dot):
                    new_fragment.update(frags[dot])
                    for key in frags[dot]:
                        frags[key] = new_fragment

    return set(imap(tuple, frags.values()))