Here is a very basic class for handling sessions on App Engine:

"""Lightweight implementation of cookie-based sessions for Google App Engine.

Classes:
Session

"""

import os
import random
import Cookie
from google.appengine.api import memcache

_COOKIE_NAME = 'app-sid'
_COOKIE_PATH = '/'
_SESSION_EXPIRE_TIME = 180 * 60


class Session(object):

    """Cookie-based session implementation using Memcached."""

    def __init__(self):
        self.sid = None
        self.key = None
        self.session = None
        cookie_str = os.environ.get('HTTP_COOKIE', '')
        self.cookie = Cookie.SimpleCookie()
        self.cookie.load(cookie_str)
        if self.cookie.get(_COOKIE_NAME):
            self.sid = self.cookie[_COOKIE_NAME].value
            self.key = 'session-' + self.sid
            self.session = memcache.get(self.key)
        if self.session:
            self._update_memcache()
        else:
            self.sid = str(random.random())[5:] + str(random.random())[5:]
            self.key = 'session-' + self.sid
            self.session = dict()
            memcache.add(self.key, self.session, _SESSION_EXPIRE_TIME)
            self.cookie[_COOKIE_NAME] = self.sid
            self.cookie[_COOKIE_NAME]['path'] = _COOKIE_PATH
            print self.cookie

    def __len__(self):
        return len(self.session)

    def __getitem__(self, key):
        if key in self.session:
            return self.session[key]
        raise KeyError(str(key))

    def __setitem__(self, key, value):
        self.session[key] = value
        self._update_memcache()

    def __delitem__(self, key):
        if key in self.session:
            del self.session[key]
            self._update_memcache()
            return None
        raise KeyError(str(key))

    def __contains__(self, item):
        try:
            i = self.__getitem__(item)
        except KeyError:
            return False
        return True

    def _update_memcache(self):
        memcache.replace(self.key, self.session, _SESSION_EXPIRE_TIME)

I would like some advices on how to improve the code for better security.

Note: In the production version it will also save a copy of the session in the datastore.

Note': I know there are much more complete implementations available online though I would like to learn more about this subject so please don't answer the question with "use that" or "use the other" library.