from django.contrib.sessions.backends.base import SessionBase, CreateError

from django.core.cache import cache

class SessionStore(SessionBase):

    """

    A cache-based session store.

    """

    def __init__(self, session_key=None):

        self._cache = cache

        super(SessionStore, self).__init__(session_key)

    def load(self):

        session_data = self._cache.get(self.session_key)

        if session_data is not None:

            return session_data

        self.create()

        return {}

    def create(self):

        # Because a cache can fail silently (e.g. memcache), we don't know if

        # we are failing to create a new session because of a key collision or

        # because the cache is missing. So we try for a (large) number of times

        # and then raise an exception. That's the risk you shoulder if using

        # cache backing.

        for i in xrange(10000):

            self.session_key = self._get_new_session_key()

            try:

                self.save(must_create=True)

            except CreateError:

                continue

            self.modified = True

            return

        raise RuntimeError("Unable to create a new session key.")

    def save(self, must_create=False):

        if must_create:

            func = self._cache.add

        else:

            func = self._cache.set

        result = func(self.session_key, self._get_session(no_load=must_create),

                self.get_expiry_age())

        if must_create and not result:

            raise CreateError

    def exists(self, session_key):

        if self._cache.get(session_key):

            return True

        return False

    def delete(self, session_key=None):

        if session_key is None:

            if self._session_key is None:

                return

            session_key = self._session_key

        self._cache.delete(session_key)