Set GIF animation loops number

set_loops.py

#!/usr/bin/python3
# https://en.wikipedia.org/wiki/GIF#Animated_GIF
# The number of loops (0..65535) is located 3 bytes after the string "NETSCAPE2.0"

import sys
sys.argc = property(lambda self: len(self.argv)).__get__(sys, sys.__class__)

if ( (sys.argc == 1) or
    ((sys.argc == 2) and (sys.argv[1] == '--help'))):
    print("Set GIF animation loops number")
    print("Usage: ./set_loops.py [image.gif] [loops]")
    print("loops = 1..65535 - 0 = infinite loop")

if (len(sys.argv) == 3):
    NAB     = b'NETSCAPE2.0'
    loops   = int(sys.argv[2])
    gifFile = sys.argv[1]
    nLoops  = loops.to_bytes(2, 'little')

    f       = open(gifFile, 'r+b')
    data    = f.read(2048)         #2kB should be enough

    if (data.find(NAB) != -1):
        offset  = data.find(NAB)+13
        f.seek(offset)
        f.write(nLoops)
    f.close()

    print('Done')
    exit()