JinJa2 模板中全局变量url_for

我读了base_template为

xample 3-10. templates/base.html: favicon definition
#+BEGIN_SRC html
{% block head %}
{{ super() }} <!-- preserve the original contents -->
<link rel="shortcut icon" href="{{ url_for('static', filename='favicon.ico') }}"
    type="image/x-icon">
<link rel="icon" href="{{ url_for('static', filename='favicon.ico') }}"
    type="image/x-icon">
{% endblock %}
#+END_SRC

视图功能：

from datetime import datetime
from flask import Flask, render_template
from flask_bootstrap import Bootstrap
from flask_moment import Moment

app = Flask(__name__)

bootstrap = Bootstrap(app)
moment = Moment(app)
..
@app.route('/')
def index():
    return render_template('index.html',
                           current_time=datetime.utcnow())

至于url_for未传递到的上下文index，该如何直接在模板中使用它。

因此，搜索JinJia2文档未找到url_for实际上是从顶层导入的关键字

In [1]: from flask import url_for        

In [2]: