Comment 4 for bug 872558

           with _semaphores_lock: #part1
               if name not in _semaphores: # part1
                    _semaphores[name] = semaphore.Semaphore() # part1
            sem = _semaphores[name] # part3
            ....
            with _semaphores_lock: # part2
                if not sem.balance < 1:# part2
                    del _semaphores[name]# part2

The solution you propose does not work in the following scenario:
one thread (T1) does part1
an other one does part2
T1 use a lock no more in _semaphores