hash functions family generator in python

I'd just do something like (if you don't need thread-safety -- not hard to alter if you DO need thread safety -- and assuming a 32-bit Python version):

import random

_memomask = {}

def hash_function(n):
  mask = _memomask.get(n)
  if mask is None:
    random.seed(n)
    mask = _memomask[n] = random.getrandbits(32)
  def myhash(x):
    return hash(x) ^ mask
  return myhash