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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.