def test_empty(self):
        """
        If passed an empty input file, L{filter} writes a I{pre} tag containing
        only an end marker to the output file.
        """
        input = BytesIO(b"")
        output = BytesIO()
        filter(input, output)
        self.assertEqual(
            output.getvalue(),
            b'<pre><span class="py-src-endmarker"></span></pre>\n')

        If passed an empty input file, L{filter} writes a I{pre} tag containing
        only an end marker to the output file.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/test/test_htmlizer.py