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The acquire() and release() functions of threading.Semaphore do not make use of the try...finally suite as it would be reasonable.
They just do 
 def acquire(self, blocking=1):
 rc = False
 self.__cond.acquire()
 [...]
 self.__cond.release()
 return rc
 [...]
 def release(self):
 self.__cond.acquire()
 [...]
 self.__cond.release()
while IMO it would be appropriate to put a try: after the acquire() calls and a finally: before the release() calls so the lock is not held forever if an exception occurs.
(Feel free to use with self.__cond: instead...)
Especially when Ctrl-C is pressed while acquire() waits, the respective KeyboardInterrupt gets thrown after acquire(), breaking the respective function and the __cond is locked forever because it is never release()d.

I wonder if it is right what I wrote. After a second thought, the acquire() should come *after* try:, as well in threading.Event. Because if Ctrl-C is pressed while waiting in acquire(), a KeyboardInterrupt is thrown immediately after returning from acquire(). This lock must be release()d again in order to be consistent.

It would be simpler to use "with" indeed.
Do you want to provide a patch?

Of course.
I hope it is correct, and I hop it is ok when I change to use "with ...:" on the other places I consider it useful as well...

What version of Python did you make your patch against? It failed applying against the default branch.
You might want to make sure you are using an up-to-date source tree, see: http://docs.python.org/devguide/setup.html#getting-the-source-code 

Oops, sorry. I have re-made it; it is to be applied to 69f58be4688a.
The original one was made against the respective file of my distribution which contains Python 2.6. It can be applied to (at least) 787b969d37f0, a fact which might help backporting it to 2.x, if wanted.

Here is another patch which fits to 2.7, if desired.

LGTM. 3.x patch a little desynchronized but it is easy to update it. I'll commit it if Antoine have no objections.
Thomas Rachel, can you please submit a contributor form?
http://python.org/psf/contrib/contrib-form/
http://python.org/psf/contrib/ 

New changeset 2253b8a18bbf by Serhiy Storchaka in branch '2.7':
Issue #11714: Use 'with' statements to assure a Semaphore releases a
http://hg.python.org/cpython/rev/2253b8a18bbf
New changeset af30c5cb248f by Serhiy Storchaka in branch '3.3':
Issue #11714: Use 'with' statements to assure a Semaphore releases a
http://hg.python.org/cpython/rev/af30c5cb248f
New changeset a26df2d03989 by Serhiy Storchaka in branch 'default':
Issue #11714: Use 'with' statements to assure a Semaphore releases a
http://hg.python.org/cpython/rev/a26df2d03989 

Thank you for the patch.
'try/finally' can't be replaced by 'with' statements in the _Event class because a conditional variable should be reinitialized in _reset_internal_locks() after fork.