def test_stopProxyingUnreferencesProducer(self):
        """
        L{TransportProxyProducer.stopProxying} drops the reference to the
        wrapped L{IPushProducer} provider.
        """
        transport = StringTransport()
        proxy = TransportProxyProducer(transport)
        self.assertIdentical(proxy._producer, transport)
        proxy.stopProxying()
        self.assertIdentical(proxy._producer, None)

        L{TransportProxyProducer.stopProxying} drops the reference to the
        wrapped L{IPushProducer} provider.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_newclient.py