使用队列交换对象，会先输出the queue is empty，然后跳出Consumer。

Python版本：3.6.7
运行结果：

Process Producer : item 160 appended to queue Producer-13
the queue is empty
The size of queue is 1
Process Producer : item 8 appended to queue Producer-13
The size of queue is 2
Process Producer : item 219 appended to queue Producer-13
The size of queue is 3
Process Producer : item 211 appended to queue Producer-13
The size of queue is 4
Process Producer : item 25 appended to queue Producer-13
The size of queue is 5