How to serialize hash objects in Python

Question

How can I serialize hash objects?, I'm using shelve to store a lot of objects.

Hierarchy:

- user
    - client
    - friend

user.py:

import time
import hashlib
from localfile import localfile

class user(object):
    _id = 0
    _ip = "127.0.0.1"
    _nick = "Unnamed"
    _files = {}
    def __init__(self, ip="127.0.0.1", nick="Unnamed"):
        self._id = hashlib.sha1(str(time.time()))
        self._ip = ip
        self._nick = nick
    def add_file(self, localfile):
        self._files[localfile.hash] = localfile
    def delete_file(self, localfile):
        del self._files[localfile.hash]

if __name__ == "__main__":
    pass

client.py:

from user import user
from friend import friend

class client(user):
    _friends = []
    def __init__(self, ip="127.0.0.1", nick="Unnamed"):
        user.__init__(self, ip, nick)
    @property
    def friends(self):
        return self._friends
    @friends.setter
    def friends(self, value):
        self._friends = value
    def add_friend(self, client):
        self._friends.append(client)
    def delete_friend(self, client):
        self._friends.remove(client)

if __name__ == "__main__":
    import shelve

    x = shelve.open("localfile", 'c')

    cliente = client()
    cliente.add_friend(friend("127.0.0.1", "Amigo1"))
    cliente.add_friend(friend("127.0.0.1", "Amigo2"))

    x["client"] = cliente
    print x["client"].friends

    x.close()

Errors:

facon@facon-E1210:~/Documentos/workspace/test$ python client.py
Traceback (most recent call last):
  File "client.py", line 28, in <module>
    x["client"] = cliente
  File "/usr/lib/python2.7/shelve.py", line 132, in __setitem__
    p.dump(value)
  File "/usr/lib/python2.7/copy_reg.py", line 70, in _reduce_ex
    raise TypeError, "can't pickle %s objects" % base.__name__
TypeError: can't pickle HASH objects

EDITED

Added user.py.