def render(self):
        """Render a single frame in a traceback."""
        return FRAME_HTML % {
            'id': self.id,
            'filename': escape(self.filename),
            'lineno': self.lineno,
            'function_name': escape(self.function_name),
            'current_line': escape(self.current_line.strip())
        }
Render a single frame in a traceback.