阅读《Flask web development》这本书表单这一章的时候，产生了一个疑问。貌似类似引用功能的代码，一个用了extend,一个用import。想知道他们的区别在哪里。刚好一好友也有此疑问，遂上stackoverflow上去提了问，今早就收到了大牛的回答，很是惊喜，这里跟大家分享下。

what are the differences between import and extends in Flask?
I am reading 《Flask web development》. in Example 4-3，

{% extends "base.html" %}
{% import "bootstrap/wtf.html" as wtf %}

I’d like to know:
What are the differences between extends and import?(I think they are quite similar in usage.)
In which situation,I will use extends or import?

Sean Vieira大神的回复：
When you extend another template the template controls you (the called controls the caller) – only named blocks in the “parent” template will be rendered:

{% codeblock %}
{% extends "base.html" %}
{% block main_content %}

Only shows up if there is a block called main_content in base.html.

{% endblock main_content%}

On the other hand an import simply binds the template to a name in your template’s scope, and you control when and where to call it (the caller controls the called):

{% import "bootstrap/wtf.html" as wtf %}

Some of your own template code with ‘wtf.calls()’ where it makes sense.

因为英语太渣，开始还不是很能理解。后来跟朋友讨论，大致明白了大神的意思。
简单翻一下就是：
extend使用base.html中规定好的字段，并只能够对其中规定的这些字段内容进行修改，所谓“the called controls the caller”.
import则用户可以选择其中的字段进行使用，是由用户对调用的模板进行控制，所谓“the caller controls the called”。

感想：第一次使用stackoverflow提问，近距离接收大牛回答，真的好激动！