title: Flask template extending issue
body: <p>Flask template extension doesn't work for some reason.</p>
  
  <p>I've created a basic flask app with the following file structure:<br>
  <a href="https://i.stack.imgur.com/MvEFV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MvEFV.png" alt="enter image description here"></a></p>
  
  <p><code>__init__.py</code> is empty</p>
  
  <p>The code from <code>app.py</code> is:</p>
  
  <pre><code>from flask import Flask, render_template
  
  app = Flask(__name__)
  
  @app.route('/')
  def index():
      return render_template("index.html")
  
  if (__name__ == "__main__"):
      app.run()
  </code></pre>
  
  <p>The code from <code>index.html</code> is:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html lang="en"&gt;
  &lt;head&gt;
    &lt;meta charset="UTF-8"&gt;
    &lt;meta name="viewport" content="width=device-width, initial-scale=1.0"&gt;
    &lt;meta http-equiv="X-UA-Compatible" content="ie=edge"&gt;
    &lt;title&gt;Document&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
  
    &lt;h1&gt;from index&lt;/h1&gt;
  
    {% block ext %}
    {% endblock %}
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>The code from <code>ext.html</code> is:</p>
  
  <pre><code>{% extends "index.html" %}
  
  {% block ext %}
  
    &lt;h2&gt;from ext&lt;/h2&gt;
  
  {% endblock %}
  </code></pre>
  
  <p>When I run in command line <code>python app.py</code> it gives no errors or warnings, logs are just</p>
  
  <pre><code> * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
  127.0.0.1 - - [27/Sep/2017 14:48:40] "GET / HTTP/1.1" 200 -
  </code></pre>
  
  <p>But in the browser, the only output I have is "from index". So Flask doesn't see template extension from ext.html, and I don't see "from ext" in the browser.  What's wrong here? Is there a typo somewhere?</p>