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 Alex Martelli approach of setting up a communications channel is the appropriate one. I would use a multiprocessing.connection.Listener to create a listener, in your choice. Documentation at: http://docs.python.org/library/multiprocessing.html#multiprocessing-listeners-clients

Rather than using AF_INET (sockets) you may elect to use AF_UNIX for Linux and AF_PIPE for Windows. Hopefully a small "if" wouldn't hurt.

Edit: I guess an example wouldn't hurt. It is a basic one, though.

#!/usr/bin/env python

from multiprocessing.connection import Listener, Client
import socket
from array import array
from sys import argv

def myloop(address):
    try:
        listener = Listener(*address)
        conn = listener.accept()
        serve(conn)
    except socket.error, e:
        conn = Client(*address)
        conn.send('this is a client')
        conn.send('close')

def serve(conn):
    while True:
        msg = conn.recv()
        if msg.upper() == 'CLOSE':
            break
        print msg
    conn.close()

if __name__ == '__main__':
    address = ('/tmp/testipc', 'AF_UNIX')
    myloop(address)

This works on OS X, so it needs testing with both Linux and (after substituting the right address) Windows. A lot of caveats exists from a security point, the main one being that conn.recv unpickles its data, so you are almost always better of with recv_bytes.