@contextmanager
def lock(file_name, block=False):
    fp = open(file_name, 'w')

    opts = fcntl.LOCK_EX
    if not block:
        opts |= fcntl.LOCK_NB

    try:
        fcntl.lockf(fp, opts)
    except IOError:
        if block:
            raise
        yield False
    else:
        try:
            yield True
        finally:
            fcntl.lockf(fp, fcntl.LOCK_UN)
            fp.close()