It's odd that the import mechanism is abused here to make objects "out of thin air". The fact that "from pbs import ffmpeg" works only if ffmpeg is actually on the path is somewhat surprising.

I think the more comfortable (and Pythonic?) way to do this would be to explicitly create these command objects:

   >>> import pbs
   >>> ffmpeg = pbs.Command('ffmpeg') # or '/usr/bin/ffmpeg', perhaps
   >>> result = ffmpeg(...)

[Edit: I really like the concept of using Python's positional and keyword arguments to construct a shell command, though. Great insight there.]