def test_elementUsingSlots(self):
        """
        An Element which is returned by the render method of another Element is
        rendered properly.
        """
        class RenderfulElement(Element):
            @renderer
            def renderMethod(self, request, tag):
                return tag.fillSlots(test2='world.')
        element = RenderfulElement(loader=XMLString(
            '<p xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1"'
            ' t:render="renderMethod">'
            '<t:slot name="test1" default="Hello, " />'
            '<t:slot name="test2" />'
            '</p>'
        ))
        return self.assertFlattensTo(element, b"<p>Hello, world.</p>")

        An Element which is returned by the render method of another Element is
        rendered properly.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_template.py