code: def _generate_html_diff(self, expected_fn, expected_lines, obtained_fn, obtained_lines):
          
          import difflib
          differ = difflib.HtmlDiff()
          return differ.make_file(
              fromlines=expected_lines,
              fromdesc=expected_fn,
              tolines=obtained_lines,
              todesc=obtained_fn,
          )
docs: Returns a nice side-by-side diff of the given files, as a string.