Losslessly rotate a GIF's hue 180 degrees

gifpalettes.py

#!/usr/bin/env python
import colorsys
import struct
import sys

f_in = open(sys.argv[1], 'rb')
f_out = open(sys.argv[2], 'wb')
# check header
if f_in.read(3) != 'GIF':
    raise TypeError('not a GIF')
f_out.write('GIF')
# check version
version = f_in.read(3)
if version not in ('87a', '89a'):
    raise TypeError('invalid GIF version')
f_out.write(version)
# discard screen width and height
f_out.write(f_in.read(4))
# get byte 10
b10 = f_in.read(1)
f_out.write(b10)
b10 = ord(b10)
# global color table bit
has_gct = b10 >> 7
if not has_gct:
    raise TypeError('no global color table')
# global color table size
gctsize = b10 % 8
gctlen = 2 ** (gctsize + 1)
# discard background color index and pixel aspect ratio
f_out.write(f_in.read(2))
# get global color table
gct = f_in.read(gctlen * 3)
for i in xrange(gctlen):
    # parse color to RGB
    color = [ord(byte) / 256. for byte in gct[i * 3:(i + 1) * 3]]
    # convert to HSV
    color_hsv = list(colorsys.rgb_to_hsv(*color))
    # rotate hue 180 degrees
    color_hsv[0] = (color_hsv[0] + .5) % 1
    # convert back to RGB
    color = colorsys.hsv_to_rgb(*color_hsv)
    f_out.write(''.join([chr(int(c * 256)) for c in color]))
# output remainder of file
f_out.write(f_in.read())
# close files
f_out.close()
f_in.close()