how to use Flask Jinja2 url_for with multiple parameters

According to url_for documentation:

If the value of a query argument is None, the whole pair is skipped.

Make sure that url_title is not None.

Or specify default value for url_title in the article_page function.

You missed a dot, try that:

url_for('.article_page', article_id=article.id, url_title=article.url_title)