Inheritance

The most powerful – and thus the most complex – part of Maile’s template engine is template inheritance. Template inheritance allows you to build a base “skeleton” template that contains all the common elements of your site and defines blocks that child templates can override.

It’s easiest to understand template inheritance by starting with an example:

<!DOCTYPE html>
<html lang="en">
<body>
    <div id="top">
        {% block top %}
            <h1>Welcome to my newsletter</h1>
        {% endblock %}
    </div>

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

This template, which we’ll call base, defines a simple HTML skeleton document that you might use for a simple two-column page. It’s the job of “child” templates to fill the empty blocks with content.

In this example, the block tag defines three blocks that child templates can fill in. All the block tag does is to tell the template engine that a child template may override those portions of the template.

A child template might look like this:

{% extends "base" %}

{% block title %}<h1>My amazing newsletter</h1>{% endblock %}

{% block content %}
<h4>From the blog</h4>
{% for entry in blog_entries %}
    <h2>{{ entry.title }}</h2>
    <p>{{ entry.body }}</p>
{% endfor %}
{% endblock %}

The extends tag is the key here. It tells the template engine that this template “extends” another template. When the template system evaluates this template, first it locates the parent – in this case, “base”.

At that point, the template engine will notice the three block tags in base.html and replace those blocks with the contents of the child template. Depending on the value of blog_entries, the output might look like:

<!DOCTYPE html>
<html lang="en">
<body>
    <div id="sidebar">
        <h1>My amazing newsletter</h1>
    </div>

    <div id="content">
        <h4>From the blog</h4>
        <h2>Entry one</h2>
        <p>This is my first entry.</p>

        <h2>Entry two</h2>
        <p>This is my second entry.</p>
    </div>
</body>
</html>

Note that since the child template didn’t define the sidebar block, the value from the parent template is used instead. Content within a {% block %} tag in a parent template is always used as a fallback.

You can use as many levels of inheritance as needed. One common way of using inheritance is the following three-level approach:

  • Create a base template that holds the main look-and-feel of your site.
  • Create a base_SECTIONNAME template for each “section” of your site. For example, base_news, base_sports. These templates all extend base.html and include section-specific styles/design.
  • Create individual templates for each type of page, such as a news article or blog entry. These templates extend the appropriate section template.

This approach maximizes code reuse and makes it easy to add items to shared content areas, such as section-wide navigation.

Here are some tips for working with inheritance:

  • If you use {% extends %} in a template, it must be the first template tag in that template. Template inheritance won’t work, otherwise.

  • More {% block %} tags in your base templates are better. Remember, child templates don’t have to define all parent blocks, so you can fill in reasonable defaults in a number of blocks, then only define the ones you need later. It’s better to have more hooks than fewer hooks.

  • If you find yourself duplicating content in a number of templates, it probably means you should move that content to a {% block %} in a parent template.

  • If you need to get the content of the block from the parent template, the {{ block.super }} variable will do the trick. This is useful if you want to add to the contents of a parent block instead of completely overriding it. Data inserted using {{ block.super }} will not be automatically escaped (see the next section), since it was already escaped, if necessary, in the parent template.

Variables created outside of a {% block %} using the template tag as syntax can’t be used inside the block. For example, this template doesn’t render anything:

{% trans "Title" as title %}
{% block content %}{{ title }}{% endblock %}
For extra readability, you can optionally give a name to your {% endblock %} tag. For example:

{% block content %}
...
{% endblock content %}

In larger templates, this technique helps you see which {% block %} tags are being closed.

Finally, note that you can’t define multiple block tags with the same name in the same template. This limitation exists because a block tag works in “both” directions. That is, a block tag doesn’t just provide a hole to fill – it also defines the content that fills the hole in the parent. If there were two similarly-named block tags in a template, that template’s parent wouldn’t know which one of the blocks’ content to use.