python linux file lock

file_lock.py

def unsafe_function():
    print('ONLY RUN ME ONCE!')

first = True
lock = open('foo', 'w+')
while True:
    try:
        fcntl.flock(lock, fcntl.LOCK_EX | fcntl.LOCK_NB)
        if first:
            unsafe_function()
        fcntl.flock(x, fcntl.LOCK_UN)
        break
    except IOError as e:
        # raise on unrelated IOErrors
        if e.errno != errno.EAGAIN:
            raise
        else:
            first = False
            time.sleep(0.1)