block

Blocks are used for inheritance and act as placeholders and replacements at the same time. They are documented in detail in the documentation for the extends tag. Block names should consist of alphanumeric characters, and underscores. Dashes are not permitted.

How do blocks work?

A block provides a way to change how a certain part of a template is rendered but it does not interfere in any way with the logic around it.

Let's take the following example to illustrate how a block works and more importantly, how it does not work:

{# base.html #}

{% for post in posts %}
  {% block post %}
    <h1>{{ post.title }}</h1>
    <p>{{ post.body }}</p>
  {% endblock %}
{% endfor %}

If you render this template, the result would be exactly the same with or without the block tag. The block inside the for loop is just a way to make it overridable by a child template:

{# child.html #}

{% extends 'base.html' %}

{% block post %}
  <article>
    <header>{{ post.title }}</header>
    <section>{{ post.text }}</section>
  </article>
{% endblock %}

Now, when rendering the child template, the loop is going to use the block defined in the child template instead of the one defined in the base one; the executed template is then equivalent to the following one:

{% for post in posts %}
  <article>
    <header>{{ post.title }}</header>
    <section>{{ post.text }}</section>
  </article>
{% endfor %}

Let's take another example: a block included within an if statement:

{% if posts is empty %}
  {% block head %}
    {{ parent() }}

    <meta name="robots" content="noindex, follow">
  {% endblock head %}
{% endif %}

Contrary to what you might think, this template does not define a block conditionally; it just makes overridable by a child template the output of what will be rendered when the condition is true. If you want the output to be displayed conditionally, use the following instead:

{% block head %}
  {{ parent() }}

  {% if posts is empty %}
    <meta name="robots" content="noindex, follow">
  {% endif %}
{% endblock head %}