Since there is a set_user(self, user)
why not have
    def get_user(self):
         return self.user


Also why not have included into Session class:
    def __setitem__(self,name,value):
        self._items[name]=value
        self._has_info=True    #we have info!

    def __getitem__(self,name):
         if self._items.has_key(name):
                 return self._items[name]
         return None

    def get(self,name,defaultvalue=''):
        value = self[name]
        if value is None:
               value = defaultvalue
        return value


Ioan