def test_explicitChainClearedWhenResolved(self):
        """
        Any recorded chaining is cleared once the chaining is resolved, since
        it no longer exists.

        In other words, if one L{Deferred} is recorded as depending on the
        result of another, and I{that} L{Deferred} has fired, then the
        dependency is resolved and we no longer benefit from recording it.
        """
        a = defer.Deferred()
        b = defer.Deferred()
        b.chainDeferred(a)
        b.callback(None)
        self.assertIsNone(a._chainedTo)

        Any recorded chaining is cleared once the chaining is resolved, since
        it no longer exists.

        In other words, if one L{Deferred} is recorded as depending on the
        result of another, and I{that} L{Deferred} has fired, then the
        dependency is resolved and we no longer benefit from recording it.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_defer.py