os.system call problem

[questions?]

I use os.system to call a display program, e.g.

os.system(displayblah blah) to call display. the program starts and
display things I wanted. When I kill display, somehow the python
program don't understand I want to move on, keep calling
os.system(displayblah blah) again and again.
I have to kill terminal to quit them all.

How can I migrate back to my python program after I kill the display
program?

Thanks a lot