跳转问题，代码检查多次，但是运行时上下文还是显示和跳转到第一篇文章

def get_details_page(request, article_id):
    all_article = Article.objects.all()
    curr_article = None
    previous_index = 0
    next_index = 0
    previous_article = None
    next_article = None
    for index, article in enumerate(all_article):
        if index == 0:
            previous_index == 0
            next_index == index + 1
        elif index == len(all_article) - 1:
            previous_index == index - 1
            next_index == index
        else:
            previous_index == index - 1
            next_index == index + 1
        if article.article_id == article_id:
            curr_article = article
            previous_article = all_article[previous_index]
            next_article = all_article[next_index]
            break

    section_list = curr_article.content.split('\n')
    return render(request, 'blog/details.html',
        {
            'curr_article': curr_article,
            'section_list': section_list,
            'previous_article': previous_article,
            'next_article': next_article,
        }
        )

<li><a href="/blog/detail/{{ previous_article.article_id }}">上一篇:{{ previous_article.article_title }}</a></li>
<li><a href="/blog/detail/{{ next_article.article_id }}">下一篇:{{ next_article.article_title }}</a></li>

如上代码检查多次，但显示依然如图所示  ：

在django shell测试了enumerate（），结果也正常，从0到9

所以问题出现在哪里，望老师和其他同学指教一下，谢谢