Ensure that only one instance running

singleton.py

#!/usr/bin/python
# -*- coding: utf-8 -*-

import fcntl
from functools import wraps
import os


def singleton(pid_filename):
    def decorator(f):
        @wraps(f)
        def decorated(*args, **kwargs):
            pid = str(os.getgid())
            pidfile = open(pid_filename, 'a+')
            try:
                fcntl.flock(pidfile.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
            except IOError:
                raise SystemExit('Already running!')
            pidfile.seek(0)
            pidfile.truncate()
            pidfile.write(pid)
            pidfile.flush()
            pidfile.seek(0)

            ret = f(*args, **kwargs)

            try:
                pidfile.close()
            except IOError, err:
                if err.errno != 9:
                    raise
            os.remove(pid_filename)
            return ret
        return decorated
    return decorator

@singleton('/tmp/main.pid')
def main():
    pass

if __name__ == '__main__':
    main()