putting processes called by python shell script in background

[Jelle Feringa // EZCT / Paris]

James, Terry, 

Thanks for your constructive comments.
Let's see if we can come down to the heart of the matter, it seems to me
these are fundamental issues relevant in any shell script.

I'm using a for loop to render a serie of images, which are 
rendered | filtered | composited > to an image

The problem is that instead of a single pipeline 2 are being executed in
parallel. How can I construct the script such that another rendering isn't
started before the first is completed?
The program is executed using win32pipe.popen3() command.


Another question is that I haven't been able to find in the documentation
how I can run these processes in the background. Where can I find pythons
version of & ?

Cheers, and thanks for your input,

Jelle.