Except that changes what it is trying to accomplish. The goal (as I read it) is to make shell scripting more palatable in python. Needing to continually differentiate between "this is python" and "this is system" gets old very fast.

In your example, what pbs is doing is no different than a bash shell script reporting a "command not found". It isn't what I would want if I were writing a complex piece of software interacting with ffmpeg, but for a simple script to process a bunch of files in a directory, I like it.

I've tended to shy away from using Python for shell scripting because subprocess is so ugly (even its out-of-favor, crippled relative os.system is nasty), and pbs looks like it does a really great job at addressing that.