lock.py

import sys
import fcntl

LOCK_EX = fcntl.LOCK_EX
LOCK_SH = fcntl.LOCK_SH
LOCK_NB = fcntl.LOCK_NB


class Flock(object):

    def __init__(self, filename, flags=None, message=None):
        self.fp = open(filename)
        if flags is None:
            flags = LOCK_EX | LOCK_NB
        self.flags = flags
        if message is None:
            message = "%s is locked" % filename
        self.message = message

    def __enter__(self):
        try:
            fcntl.flock(self.fp.fileno(), self.flags)
        except IOError:
            sys.stderr.write(self.message + "\n")
            sys.exit(1)

    def __exit__(self, type, value, traceback):
        if type is None or type is KeyboardInterrupt:
            self.fp.close()
            return True

usage.py

"""
Lock a file.
Only works on *nix systems
"""
import sys
from lock import Flock

with Flock(__file__):
    print("Locked file. Hit enter to unlock")
    dummy = sys.stdin.readline()