trim.py

from PIL import Image

def trim(im):
    data = list(im.getdata())
    cs = set()
    w, h = im.size
    for x in range(w):
        cs.add(data[x])
        cs.add(data[(h - 1) * w + x])
    for y in range(h):
        cs.add(data[y * w])
        cs.add(data[y * w + (w - 1)])
    [c] = cs
    while True:
        if all(i == c for i in data[:w]):
            del data[:w]
            h -= 1
        elif all(i == c for i in data[(h - 1) * w:]):
            del data[(h - 1) * w:]
            h -= 1
        elif all(i == c for i in data[::w]):
            del data[::w]
            w -= 1
        elif all(i == c for i in data[w - 1::w]):
            del data[w - 1::w]
            w -= 1
        else:
            break
    out = Image.new(im.mode, (w, h))
    out.putdata(data)
    return out

if __name__ == "__main__":
    import sys

    err = False

    for f in sys.argv[1:]:
        try:
            im = Image.open(f)
            im = trim(im)
            im.save(f + ".trim.png")
        except Exception as e:
            print(e, file=sys.stderr)
            err = True

    if err:
        sys.exit(1)