    from Common import *

    class SessionStore(Object):
        """A general session store.

        SessionStores are dictionary-like objects used by Application to
        store session state. This class is abstract and it's up to the
        concrete subclass to implement several key methods that determine
        how sessions are stored (such as in memory, on disk or in a
        database).

        Subclasses often encode sessions for storage somewhere. In light
        of that, this class also defines methods encoder(), decoder() and
        setEncoderDecoder(). The encoder and decoder default to the load()
        and dump() functions of the cPickle or pickle module. However,
        using the setEncoderDecoder() method, you can use the functions
        from marshal (if appropriate) or your own encoding scheme.
        Subclasses should use encoder() and decoder() (and not
        pickle.load() and pickle.dump()).

        Subclasses may rely on the attribute self._app to point to the
        application.

        Subclasses should be named SessionFooStore since Application
        expects "Foo" to appear for the "SessionStore" setting and
        automatically prepends Session and appends Store. Currently, you
        will also need to add another import statement in Application.py.
        Search for SessionStore and you'll find the place.

        TO DO

        * Should there be a check-in/check-out strategy for sessions to
          prevent concurrent requests on the same session? If so, that can
          probably be done at this level (as opposed to pushing the burden
          on various subclasses).

        """

        ## Init ##

        def __init__(self, app):
            """ Subclasses must invoke super. """
            Object.__init__(self)
            self._app = app
            try:
                import cPickle as pickle
            except ImportError:
                import pickle
            if hasattr(pickle, 'HIGHEST_PROTOCOL'):
                def dumpWithHighestProtocol(obj, f,
                        proto=pickle.HIGHEST_PROTOCOL, dump=pickle.dump):
                    return dump(obj, f, proto)
                self._encoder = dumpWithHighestProtocol
            else:
                self._encoder = pickle.dump
            self._decoder = pickle.load

        ## Access ##

        def application(self):
            return self._app

        ## Dictionary-style access ##

        def __len__(self):
            raise AbstractError, self.__class__

        def __getitem__(self, key):
            raise AbstractError, self.__class__

        def __setitem__(self, key, item):
            raise AbstractError, self.__class__

        def __delitem__(self, key):
            """Delete an item.

            Subclasses are responsible for expiring the session as well.
            Something along the lines of:
                sess = self[key]
                if not sess.isExpired():
                    sess.expiring()

            """
            raise AbstractError, self.__class__

        def has_key(self, key):
            raise AbstractError, self.__class__

        def keys(self):
            raise AbstractError, self.__class__

        def clear(self):
            raise AbstractError, self.__class__

        def setdefault(self, key, default):
            raise AbstractError, self.__class__

        ## Application support ##

        def storeSession(self, session):
            raise AbstractError, self.__class__

        def storeAllSessions(self):
            raise AbstractError, self.__class__

        def cleanStaleSessions(self, task=None):
            """Clean stale sessions.

            Called by the Application to tell this store to clean out all
            sessions that have exceeded their lifetime.

            """
            curTime = time.time()
            for key in self.keys():
                try:
                    sess = self[key]
                except KeyError:
                    pass # session was already deleted by some other thread
                else:
                    if curTime - sess.lastAccessTime() >= sess.timeout() \
                            or sess.timeout() == 0:
                        try:
                            del self[key]
                        except KeyError:
                            pass # already deleted by some other thread

        ## Convenience methods ##

        def items(self):
            itms = []
            for k in self.keys():
                try:
                    itms.append((k, self[k]))
                except KeyError:
                    # since we aren't using a lock here, some keys
                    # could be already deleted again during this loop
                    pass
            return itms

        def values(self):
            vals = []
            for k in self.keys():
                try:
                    vals.append(self[k])
                except KeyError:
                    pass
            return vals

        def get(self, key, default=None):
            try:
                return self[key]
            except KeyError:
                return default

        ## Encoder/decoder ##

        def encoder(self):
            return self._encoder

        def decoder(self):
            return self._decoder

        def setEncoderDecoder(self, encoder, decoder):
            self._encoder = encoder
            self._decoder = decoder

        ## As a string ##

        def __repr__(self):
            d = {}
            for key, value in self.items():
                d[key] = value
            return repr(d)