def add_tag(request):
    if request.method == 'GET':
        html = html_template.format(where = request.path)
        resp = HttpResponse(html)
        return resp
    elif request.method == 'POST':
        article_id = int(request.POST['id'])
        new_tag = request.POST['new_tag']
        for article in articles_list:
            if article[0] == article_id:
                article[4].append(new_tag)
        return article_by_id(request, article_id)