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Presumably, by changing threading.Semaphore to stop using 
any code from threading.py, then threading.currentThread() 
never gets called and so a _DummyThread is never created 
then.

I expect the reason a _DummyThread causes Event leaks is 
just that Thread.__init__ always ends up allocating a Python 
lock (Thread.__block), which allocates a Windows Event 
under the covers.

It *could* be that Thread.__block is never actually used for 
dummy threads, in which case we could avoid allocating it in 
that case (or could get rid of it right way in 
_DummyThread.__init__).  The dummy thread would still clog 
the _active dict, but wouldn't leak Events then.

It's certainly true that Python has no way to know when a 
thread it didn't start goes away.