set_hash_seed.py

#!/usr/bin/python3

# Python 3 changes hash seeds upon interpreter start.
# https://docs.python.org/3/reference/datamodel.html#object.__hash__
#
# This was to fix the following vulnerability:
# http://ocert.org/advisories/ocert-2011-003.html
#
# For non-web apps, the hash non-determinism between interpreter runs
# can be fixed by setting an env var:
# export PYTHONHASHSEED=1234
# But, if the interpreter was started and the env var 
# wasn't set ahead of time, here's a fix that can be 
# done at runtime from the interpreter itself.

import os
import sys

if __name__ == '__main__':
    # Set hash seed and restart interpreter.
    # This will be done only once if the env var is clear.
    if not os.environ.get('PYTHONHASHSEED'):
        os.environ['PYTHONHASHSEED'] = '1234'
        os.execv(sys.executable, ['python3'] + sys.argv)

print(hash('foo'))

Execution example:

$ python3 --version
Python 3.5.2

$ python3 fixed_hash.py
-8715801143494849942

$ python3 fixed_hash.py
-8715801143494849942

$ PYTHONHASHSEED="random" python3 fixed_hash.py
8760842921345983771

$ PYTHONHASHSEED="random" python3 fixed_hash.py
-456391045329298245