flip.py

#!/usr/bin/env python3
"""Toggle the bit at the specified offset.
Syntax: <cmdname> filename bit-offset"""

import sys
import os
import random

fname = sys.argv[1]
# Convert bit offset to bytes + leftover bits
file_stats = os.stat(fname)
total_bytes = file_stats.st_size

total_bits = total_bytes * 8

target_bit = random.randrange(total_bits)
nbytes = target_bit // 8
nbits = target_bit % 8


# Open in read+write, binary mode; read 1 byte
fp = open(fname, "r+b")
fp.seek(nbytes, 0)
c = fp.read(1)

# Toggle bit at byte position `nbits`
toggled = bytes([ord(c) ^ (1 << nbits)])
# print(toggled) # diagnostic output

# Back up one byte, write out the modified byte
fp.seek(-1, 1)  # or absolute: fp.seek(nbytes, 0)
fp.write(toggled)
fp.close()