修复边界值问题（当刚开始就没有找到文章或者就根本被没有文章的时候，会出现上一篇按钮）

app/Article.py

@@ -74,11 +74,12 @@ def get_today_article(user_word_list, had_read_articles):
                         break
                 if result_of_generate_article == "found":  # 用于成功找到文章后及时退出外层循环
                     break
-        if result_of_generate_article != "found":  # 阅读完所有文章，或者循环3次没有找到适合的文章，则对 index+=1 进行回滚
+        if result_of_generate_article != "found":  # 阅读完所有文章，或者循环3次没有找到适合的文章，则放入空（“null”）
-            had_read_articles["index"] -= 1
+            had_read_articles["article_ids"].append('null')
     else:  # 生成已经阅读过的文章
         d = random.choice(result)
         text_level = text_difficulty_level(d['text'], d3)
+        result_of_generate_article = "found"
 
     today_article = None
     if d:

app/templates/userpage_get.html

@@ -44,14 +44,16 @@
 {#        <div class="alert alert-warning" role="alert">Congratulations! {{ message }}</div>#}
 {#    {% endfor %}#}
 
+    {% if result_of_generate_article != "had read all articles" %}
         <a id="next_btn" class="btn btn-success" href="/{{ username }}/reset" role="button"> 下一篇 Next Article </a>
-    {% if session.get('had_read_articles') != None and session.get('had_read_articles')["index"] !=0 %}
+    {% endif %}
+    {% if session.get('had_read_articles') and session.get('had_read_articles')['index']>0 %}
         <a id="pre_btn" class="btn btn-success" href="/{{ username }}/back" role="button"> 上一篇 Previous Article </a>
     {% endif %}
 
     <p><b>阅读文章并回答问题</b></p>
     <div id="text-content">
-        {% if today_article %}
+        {% if result_of_generate_article == 'found' %}
             <div class="alert alert-success" role="alert">According to your word list, your level is <span class="badge bg-success">{{ today_article["user_level"] }}</span>  and we have chosen an article with a difficulty level of <span class="badge bg-success">{{ today_article["text_level"] }}</span> for you.</div>
                 <p class="text-muted">Article added on: {{ today_article["date"] }}</p><br/>
             <div class="p-3 mb-2 bg-light text-dark"><br/>

app/user_service.py

@@ -31,6 +31,9 @@ def user_reset(username):
     '''
     if request.method == 'GET':
         had_read_articles = session.get("had_read_articles")
+        if had_read_articles['article_ids'][-1] == "null":  # 如果当前还是“null”，则将“null”pop出来,无需index+=1
+            had_read_articles['article_ids'].pop()
+        else:  # 当前不为“null”，直接 index+=1
             had_read_articles["index"] += 1
         session["had_read_articles"] = had_read_articles
         return redirect(url_for('user_bp.userpage', username=username))
@@ -45,9 +48,10 @@ def user_back(username):
     :return: 返回页面内容
     '''
     if request.method == 'GET':
-        if session.get("found_article"):
         had_read_articles = session.get("had_read_articles")
-            had_read_articles["index"] -= 1
+        had_read_articles["index"] -= 1  # 上一篇，index-=1
+        if had_read_articles['article_ids'][-1] == "null":  # 如果当前还是“null”，则将“null”pop出来
+            had_read_articles['article_ids'].pop()
         session["had_read_articles"] = had_read_articles
         return redirect(url_for('user_bp.userpage', username=username))
 
@@ -137,10 +141,6 @@ def userpage(username):
             words += x[0] + ' '
         had_read_articles, today_article, result_of_generate_article = get_today_article(user_freq_record, session.get('had_read_articles'))
         session['had_read_articles'] = had_read_articles
-        if today_article is None:
-            session["found_article"] = False
-        else:
-            session["found_article"] = True
         # 通过 today_article，加载前端的显示页面
         return render_template('userpage_get.html',
                                username=username,