菜单是通过class="active"来修改状态的。

在Flask中的实现方式：

<nav class="blog-nav">
  <a id="allposts"  class="blog-nav-item" href="{{ url_for('allposts')}}">All Posts</a>
  <a id="index"     class="blog-nav-item" href="{{ url_for('index')}}">Index</a>
  <a id="favorites" class="blog-nav-item" href="{{ url_for('favorites')}}">Favorites</a>
</nav>

At the bottom of your base/layout template just add this

<script>
  $(document).ready(function () {
  $("#{{request.endpoint}}").addClass("active"); })
</script>

and the right elements will be set active.

EDIT: If you have a layout with elements in a list, like this:

<nav class="blog-nav">
  <ul class="nav navbar-nav">
    <li>
        <a id="allposts"  class="blog-nav-item" href="{{ url_for('allposts')}}">All Posts</a>
    </li>
    <li>
        <a id="index"     class="blog-nav-item" href="{{ url_for('index')}}">Index</a>
    </li>
    <li>
        <a id="favorites" class="blog-nav-item" href="{{ url_for('favorites')}}">Favorites</a>
    </li>
  </ul>
</nav>

use the **parent() **function to get the li element instead of the link.

<script>
    $(document).ready(function () {
    $("#{{request.endpoint}}").parent().addClass("active"); })
</script>