can a python script know that another instance of the same script is running… and then talk to it?

Question

I\'d like to prevent multiple instances of the same long-running python command-line script from running at the same time, and I\'d like the new instance to be able to send data

Answer 1

The general approach is to have the script, on startup, set up a communication channel in a way that's guaranteed to be exclusive (other attempts to set up the same channel fail in a predictable way) so that further instances of the script can detect the first one's running and talk to it.

Your requirements for cross-platform functionality strongly point towards using a socket as the communication channel in question: you can designate a "well known port" that's reserved for your script, say 12345, and open a socket on that port listening to localhost only (127.0.0.1). If the attempt to open that socket fails, because the port in question is "taken", then you can connect to that port number instead, and that will let you communicate with the existing script.

If you're not familiar with socket programming, there's a good HOWTO doc here. You can also look at the relevant chapter in Python in a Nutshell (I'm biased about that one, of course;-).