在Python中，不使用/ proc文件系统，如何判断给定的PID是否正在运行？

Say I have a PID, like 555. I want to see if that pid is running or has completed. I can check /proc/ but I don't have access to that in my production environment. What's the best way to do this, short of something hackish like opening a pipe to "ps"?Say I have a PID, like 555. I want to see if th