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Ah, I didn't notice that Semaphore uses a Condition lock which uses an 
RLock which calls currentThread before and gets a lock from 
thread.allocate_lock which probably uses an Event on Windows.  I also 
noticed that if __debug__ is set than the _note method uses it as well.

It looks like Thread.__block can't be called for a _DummyThread since 
the only places self.__block is used is in Thread.__stop (which is called 
in Thread.__bootstrap which is called by Thread.start which will raise an 
AssertionError since Thread.__started will be set to True thanks to 
_DummyThread.__init__) and in Thread.join which is overridden in 
_DummyThread.  So it looks like deleting the key should be safe in 
_DummyThread.__init__.

Probably wouldn't hurt to delete self.__stderr while we are at it since it 
never gets used either and thus is basically a ref leak.  Sound good to 
you, Tim?