Random bit flipping

gistfile1.py

# Assuming Python >=2.5, <3.0
import os
from random import randint
num_random_bits = 9
with open('f', 'r+') as f:
    f.seek(0, os.SEEK_END)
    length = f.tell()-1
    for _ in range(num_random_bits):
        place = randint(0, length)
        f.seek(place, os.SEEK_SET)
        old_byte = ord(f.read(1))
        random_bit = 2**randint(0,7)
        updated_byte = chr(old_byte ^ random_bit)
        f.seek(place, os.SEEK_SET)
        f.write(updated_byte)