*bsd/linux lockfile+pidfile as contextmanager

lockme.py

#!/usr/bin/env python

import os, fcntl, time
from contextlib import contextmanager

@contextmanager
def lockfile(filename):
    with open(filename,'a+') as lf:
        try:
            fcntl.flock(lf.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)
        except IOError:
            raise SystemExit('Cannot acquire lock')
        else:
            lf.truncate(0)
            lf.write('%d\n' % os.getpid())
            lf.flush()
            try:
                yield
            finally:
                lf.truncate(0)

def job(foo):
    print "Doing serious business"
    print foo
    time.sleep(15)
    print "Bye"

if __name__ == "__main__":
    with lockfile('LOCK'):
        job('yo, dawg!')