def test_installingOldStyleResolver(self):
        """
        L{PluggableResolverMixin} will wrap an L{IResolverSimple} in a
        complexifier.
        """
        reactor = PluggableResolverMixin()
        it = SillyResolverSimple()
        verifyObject(IResolverSimple, reactor.installResolver(it))
        self.assertIsInstance(reactor.nameResolver, SimpleResolverComplexifier)
        self.assertIs(reactor.nameResolver._simpleResolver, it)

        L{PluggableResolverMixin} will wrap an L{IResolverSimple} in a
        complexifier.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_resolver.py