def test_importAll(self):
        """
        L{twisted.application.internet} dynamically defines L{service.Service}
        subclasses. This test ensures that the subclasses exposed by C{__all__}
        are valid attributes of the module.
        """
        for cls in internet.__all__:
            self.assertTrue(
                hasattr(internet, cls),
                '%s not importable from twisted.application.internet' % (cls,))

        L{twisted.application.internet} dynamically defines L{service.Service}
        subclasses. This test ensures that the subclasses exposed by C{__all__}
        are valid attributes of the module.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_application.py