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_DummyThread.__init__() explicitly deletes self._Thread__block:
 def __init__(self):
 Thread.__init__(self, name=_newname("Dummy-%d"))
 # Thread.__block consumes an OS-level locking primitive, which
 # can never be used by a _DummyThread. Since a _DummyThread
 # instance is immortal, that's bad, so release this resource.
 del self._Thread__block
 ^^^^^^^^^^^^^^^^^^^^^^^