def setUp(self):
        """
        Reduce the CPython check interval so that thread switches happen much
        more often, hopefully exercising more possible race conditions.  Also,
        delay actual test startup until the reactor has been started.
        """
        if _PY3:
            if getattr(sys, 'getswitchinterval', None) is not None:
                self.addCleanup(sys.setswitchinterval, sys.getswitchinterval())
                sys.setswitchinterval(0.0000001)
        else:
            if getattr(sys, 'getcheckinterval', None) is not None:
                self.addCleanup(sys.setcheckinterval, sys.getcheckinterval())
                sys.setcheckinterval(7)

        Reduce the CPython check interval so that thread switches happen much
        more often, hopefully exercising more possible race conditions.  Also,
        delay actual test startup until the reactor has been started.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_threadable.py