Get the brightness of hex color

gistfile1.py

# e.g.
# brightness("#ffffff") = (255/255.0 + 255/255.0 + 255/255.0)/3.0 = 1.0
# brightness("#000000") = (0/255.0 + 0/255.0 + 0/255.0)/3.0 = 0.0
# brightness("#666666") = (102/255.0 + 102/255.0 + 102/255.0)/3.0 = 0.4
def brightness(color):
    rgb = struct.unpack('BBB', color[1:].decode('hex'))
    brightness = 0
    for x in rgb:
      brightness += x / 255.0
    return brightness / 3.0