Flask 烧瓶引导导航栏功能不正常

下面是我在app.py文件中的内容：

from flask import Flask, render_template
from flask_bootstrap import Bootstrap

app = Flask( __name__)
Bootstrap(app)

@app.route('/')
def hello():
    return render_template('index.html')

if __name__ == '__main__':
    app.run(debug=True)

以下是我在index.html文件中的内容：

{% extends "bootstrap/base.html" %}

{% block navbar %}
<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="#">Navbar</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>

  <div class="collapse navbar-collapse" id="navbarSupportedContent">
    <ul class="navbar-nav mr-auto">
      <li class="nav-item active">
        <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link</a>
      </li>
      <li class="nav-item dropdown">
        <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
          Dropdown
        </a>
        <div class="dropdown-menu" aria-labelledby="navbarDropdown">
          <a class="dropdown-item" href="#">Action</a>
          <a class="dropdown-item" href="#">Another action</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="#">Something else here</a>
        </div>
      </li>
      <li class="nav-item">
        <a class="nav-link disabled" href="#">Disabled</a>
      </li>
    </ul>
    <form class="form-inline my-2 my-lg-0">
      <input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
      <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
    </form>
  </div>
</nav>
{% endblock %}


{% block content %}

{% endblock %}

{%extends“bootstrap/base.html”%}
{%block navbar%}










搜寻
{%endblock%}
{%block content%}
{%endblock%}

第一张图片是我期望看到的，第二张图片是我实际得到的。我已经清除了浏览器上的所有缓存，尝试了谷歌Chrome和Mozilla Firefox。请告知。

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Flask boostrap已经死了我想最后一次犯罪是在3年前

使用该初学者模板作为base.html并在那里添加导航栏