`extends`

The extends tag is used to declare a parent template. It should be the very first tag used in a child template and a child template can only extend up to one parent template.

The best way to understand template inheritance is to study an example. Let us look at a parent template called "base":

<html>
	<head>
		<title>{% block title %} {% endblock %}</title>
	</head>
	<body>
		<div id="content">
			{% block content %}
				Default content goes here.
			{% endblock %}
		</div>

		<div id="footer">
			{% block footer %}
				Default footer content
			{% endblock %}
		</div>
	</body>
</html>

And now let's look at a child template called "home" which extends "base":

{% extends "base" %}

{% block title %} Home {% endblock %}

{% block content %}
	Home page content.
{% endblock %}

And finally let's look at the resulting output after evaluating "home":

<html>
	<head>
		<title> Home </title>
	</head>
	<body>
		<div id="content">
			Home page content will override the default content.
		</div>

		<div id="footer">
			Default footer content
		</div>
	</body>
</html>

To summarize, parent templates define blocks and child templates will override the contents of those blocks. If a child template does not override the content of a particular block, the content provided by the parent template will be used.

There is no limit to how long of an inheritance chain that you can create; i.e. a child template can itself have a child template. A lot of potential comes from this fact because you can create a hierarchy of templates to minimize how much content you have to write on the lower levels.

Dynamic Inheritance

The extends tag will accept an expression to determine the parent template at runtime. For example:

{% extends ajax ? 'ajax' : 'base' %}