之前对Daemon线程理解有偏差，特记录说明：
一、什么是Daemon
A thread can be flagged as a “daemon thread”. The significance of this flag is that the entire Python program exits when only daemon threads are left. The initial value is inherited from the creating thread. The flag can be set through the daemon property.
线程可以被标识为"Daemon线程"，Daemon线程表明整个Python主程序只有在Daemon子线程运行时可以退出。该属性值继承自父线程，可通过setDaemon()函数设定该值。A threa
