def test_chainDeferredRecordsExplicitChain(self):
        """
        When we chain a L{Deferred}, that chaining is recorded explicitly.
        """
        a = defer.Deferred()
        b = defer.Deferred()
        b.chainDeferred(a)
        self.assertIs(a._chainedTo, b)

        When we chain a L{Deferred}, that chaining is recorded explicitly.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_defer.py