use

Horizontal reuse is an advanced Canvas feature that is hardly ever used in regular templates. It is mainly used by projects that need to make template blocks reusable without using inheritance.

Template inheritance is one of the most powerful features of Canvas but it is limited to single inheritance; a template can only extend one other template. This limitation makes template inheritance simple to understand and easy to debug.

{% extends 'base.html' %}

{% block title %}{% endblock %}
{% block content %}{% endblock %}

Horizontal reuse is a way to achieve the same goal as multiple inheritance, but without the associated complexity:

{% extends 'base.html' %}

{% use 'blocks.html' %}

{% block title %}{% endblock %}
{% block content %}{% endblock %}

The use statement tells Canvas to import the blocks defined in blocks.html into the current template (it's like macros, but for blocks):

{# blocks.html #}

{% block sidebar %}{% endblock %}

In this example, the use statement imports the sidebar block into the main template. The code is mostly equivalent to the following one (the imported blocks are not outputted automatically):

{% extends 'base.html' %}

{% block sidebar %}{% endblock %}
{% block title %}{% endblock %}
{% block content %}{% endblock %}

The use tag only imports a template if it does not extend another template, if it does not define macros, and if the body is empty. But it can use other templates. Because use statements are resolved independtly of the context passed to the template, the template reference cannot be an expression.

The main template can also override any imported block. If the template already defines the sidebar block, then the one defined in blocks.html is ignored. To avoid name conflicts, you can rename imported blocks:

{% extends 'base.html' %}

{% use 'blocks.html' with sidebar as base_sidebar, title as base_title %}

{% block sidebar %}{% endblock %}
{% block title %}{% endblock %}
{% block content %}{% endblock %}

The parent() function automatically determines the correct inheritance tree, so it can be used when overriding a block defined in an imported template:

{% extends 'base.html' %}

{% use 'blocks.html' %}

{% block sidebar %}
  {{ parent() }}
{% endblock %}

{% block title %}{% endblock %}
{% block content %}{% endblock %}

In this example, the parent() function will correctly call the sidebar block from the blocks.html template.

Renaming allows you to simulate inheritance by calling the "parent" block:

{% extends 'base.html' %}

{% use 'blocks.html' with sidebar as parent_sidebar %}

{% block sidebar %}
  {{ block('parent_sidebar') }}
{% endblock %}

You can use as many use statements as you want in any given template. If two imported templates define the same block, the latest one wins.