Create a lockfile and writes PID

lockfile.py

import os
import sys

fname = "./lockfile"
if os.path.isfile(fname):
    os.remove(fname)

try:
    a = os.open(fname, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0644)
    os.write(a, str(os.getpid()) + '\n')
    os.close(a)
except OSError:
    sys.stderr.write("Another instance of this program is running\n")
    sys.exit(1)