title: what are the differences between import and extends in Flask?
body: <p>I am reading 《Flask web development》.
  in Example 4-3，</p>
  
  <pre><code>{% extends "base.html" %}
  {% import "bootstrap/wtf.html" as wtf %}
  </code></pre>
  
  <p>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?</p>