Best way to launch a process and block until it is finished
<p>I'm told subprocess.Popen does not block, so the process runs in the background and your python program does not wait until it has finished.  However, this is not what I want.</p>

<p>I'm having my program call into an external python program, which I have no control over.  It does not return any return codes, just runs, operates on files, and finishes.  I want to call this and only continue my python program when that call has completed.  How do I do this?</p>
