Python中的事件驱动系统调用

I'm trying to implement an event driven process with system call or subprocess. Basically I want to launch a non-blocking system command and upon completion of that system call, I want a function to be called. This is so that I can start a GUI progress bar, launch a system command and have the progress bar continue, and when the system call finishes, have the progress bar stop. I'm trying to implement an event driven process