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.