def sourceLines(self, request, tag):
        """
        Render the source line indicated by C{self.frame} and several
        surrounding lines.  The active line will be given a I{class} of
        C{"snippetHighlightLine"}.  Other lines will be given a I{class} of
        C{"snippetLine"}.
        """
        for (lineNumber, sourceLine) in self._getSourceLines():
            newTag = tag.clone()
            if lineNumber == self.frame[2]:
                cssClass = "snippetHighlightLine"
            else:
                cssClass = "snippetLine"
            loader = TagLoader(newTag(**{"class": cssClass}))
            yield _SourceLineElement(loader, lineNumber, sourceLine)

        Render the source line indicated by C{self.frame} and several
        surrounding lines.  The active line will be given a I{class} of
        C{"snippetHighlightLine"}.  Other lines will be given a I{class} of
        C{"snippetLine"}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/util.py