def test_passesThroughDeferreds(self):
        """
        L{defer.ensureDeferred} will pass through a Deferred unchanged.
        """
        d = defer.Deferred()
        d2 = defer.ensureDeferred(d)
        self.assertIs(d, d2)

        L{defer.ensureDeferred} will pass through a Deferred unchanged.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_defer.py