'Ensure that a given queue has been declared, and then call
the callback with no arguments.'

def ensure_queue(self, queue_name, callback):

    if (not self.connection.is_open):
        self._connect()
    if (queue_name not in self.queues):
        self.channel.queue_declare(queue=queue_name, durable=True)
        self.queues.add(queue_name)
    callback()
