[Tutor] Allow only one instance of a process

[Roger]

On Sunday 17 May 2009 21:54:54 Kent Johnson wrote:
> On Sat, May 16, 2009 at 10:26 PM, Sylvain Ste-Marie
>
> <stemarie.sylvain at gmail.com> wrote:
> > I'm currently writing a script to batch convert video for my psp
> >
> > It basically looks into a folder for video and launch ffmpeg:
> >
> > ffmpeg -i "videoname" -f psp -r 29.97 -b 768k -ar 24000 -ab 64k -s
> > 368x208 "videoname.mp4"
> >
> > my idea is basically to check for pid but how do i do that?
>
> Look at psutil:
> http://code.google.com/p/psutil/
>
> But you will have a race condition if you do something like this:
>
> if ffmpeg is not running:
>   start ffmpeg
>
> A file lock is a better solution. Google 'python file lock' for recipes.
>

As a Java programmer just starting with Python, this answer surprised me. I
would've been googling for the Python equivalent of the Singleton pattern. 
I guess it's going to take longer than I thought to get my head around the
differences. 

With the lock file solution, if your program crashes, don't you have to 
undertake some form of manual/automated recovery to remove any left
over lock files?

Regards