Python fcntl does not lock as expected

Question

On a Debian-based OS (Ubuntu, Debian Squeeze), I'm using Python (2.7, 3.2) fcntl to lock a file. As I understand from what I read, fnctl.flock locks a file in a way, that an exception will be thrown if another client wants to lock the same file.

I built a little example, which I would expect to throw an excepiton, since I first lock the file, and then, immediately after, I try to lock it again:

#!/usr/bin/env python # -*- coding: utf-8 -*- import fcntl fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX) try:     fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX | fcntl.LOCK_NB) except IOError:     print("can't immediately write-lock the file ($!), blocking ...") else:     print("No error") 

But the example just prints "No error".

If I split this code up to two clients running at the same time (one locking and then waiting, the other trying to lock after the first lock is already active), I get the same behavior - no effect at all.

Whats the explanation for this behavior?

EDIT:

Changes as requested by nightcracker, this version also prints "No error", although I would not expect that:

#!/usr/bin/env python # -*- coding: utf-8 -*- import fcntl import time fcntl.flock(open('/tmp/locktest', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB) try:     fcntl.flock(open('/tmp/locktest', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB) except IOError:     print("can't immediately write-lock the file ($!), blocking ...") else:     print("No error") 

Answer 1

Old post, but if anyone else finds it, I get this behaviour:

>>> fcntl.flock(open('test.flock', 'w'), fcntl.LOCK_EX) >>> fcntl.flock(open('test.flock', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB) # That didn't throw an exception  >>> f = open('test.flock', 'w') >>> fcntl.flock(f, fcntl.LOCK_EX) >>> fcntl.flock(open('test.flock', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB) Traceback (most recent call last):   File "<stdin>", line 1, in <module> IOError: [Errno 35] Resource temporarily unavailable >>> f.close() >>> fcntl.flock(open('test.flock', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB) # No exception 

It looks like in the first case, the file is closed after the first line, presumably because the file object is inaccessible. Closing the file releases the lock.