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