def test_defaultExtension(self):
        """
        L{FilePath.setContent} creates temporary files with the extension
        I{.new} if no alternate extension value is given.
        """
        fp = TrackingFilePath(self.mktemp())
        fp.setContent(b"hello")
        self._assertOneOpened(fp, b".new")

        L{FilePath.setContent} creates temporary files with the extension
        I{.new} if no alternate extension value is given.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_paths.py