def len(self, dict_name, priority_min='-inf', priority_max='+inf'):
        '''Get the number of items in (part of) a dictionary.

        Returns number of items in `dict_name` within
        [`priority_min`, `priority_max`].  This is similar to
        ``len(filter(dict_name, priority_min, priority_max))`` but
        does not actually retrieve the items.

        This is a read-only operation that does not require or
        honor a session lock.

        :param str dict_name: dictionary name to query
        :param float priority_min: lowest priority score
        :param float priority_max: highest priority score
        '''
        dict_name = self._namespace(dict_name)
        conn = redis.Redis(connection_pool=self.pool)
        return conn.zcount(dict_name + 'keys', priority_min, priority_max)