python 内存回收

refer to,

https://saucer-man.com/operation_and_maintenance/298.html

https://www.cnblogs.com/geaozhang/p/7111961.html

https://www.jianshu.com/p/16701cdba23c

https://blog.csdn.net/nirendao/article/details/44426201

https://blog.csdn.net/jiangjiang_jian/article/details/79140742