Inject a file into another file

inject.py

import sys
import os

try:
    fn = sys.argv[1]
except IndexError:
    fn = 'LOOPTEST.SND'
out_fn = 'NULL.DAT'

in_data = open(fn, 'rb').read()

if len(in_data) > os.path.getsize(out_fn):
    raise Exception('Input file larger than output file')

out_f = open(out_fn, 'r+b')
out_f.seek(0)

out_f.write(in_data)