def test_validSubdir(self):
        """
        Verify that a valid subdirectory will show up as a directory, but not as a
        file, not as a symlink, and be listable.
        """
        sub1 = self.path.child(b'sub1')
        self.assertTrue(sub1.exists(),
                        "This directory does exist.")
        self.assertTrue(sub1.isdir(),
                        "It's a directory.")
        self.assertFalse(sub1.isfile(),
                        "It's a directory.")
        self.assertFalse(sub1.islink(),
                        "It's a directory.")
        self.assertEqual(sub1.listdir(),
                             [b'file2'])

        Verify that a valid subdirectory will show up as a directory, but not as a
        file, not as a symlink, and be listable.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_paths.py