Templates¶

A template is a text file. It can generate any text-based format (HTML, XML, CSV, etc.).

A template contains variables, which get replaced with values when the template is evaluated, and tags, which control the logic of the template.

Below is a minimal template that illustrates a few basics. Each element will be explained later in this document.

{% extends "base_generic.html" %}

{% block title %}{{ section.title }}{% endblock %}

{% block content %}
<h1>{{ section.title }}</h1>

{% for story in story_list %}
<h2>
  <a href="{{ story.get_absolute_url }}">
    {{ story.headline|upper }}
  </a>
</h2>
<p>{{ story.tease|truncatewords:"100" }}</p>
{% endfor %}
{% endblock %}

Variables¶

Variables look like this: {{ variable }}. When the template engine encounters a variable, it evaluates that variable and replaces it with the result. Variable names consist of any combination of alphanumeric characters and the underscore ("_") but may not start with an underscore, and may not be a number. The dot (".") also appears in variable sections, although that has a special meaning, as indicated below. Importantly, you cannot have spaces or punctuation characters in variable names.

Use a dot (.) to access attributes of a variable.

{% for k, v in defaultdict.items %}
    Do something with k and v here...
{% endfor %}

In the above example, {{ section.title }} will be replaced with the title attribute of the section object.

If you use a variable that doesn’t exist, the template system will insert the value of the string_if_invalid option, which is set to '' (the empty string) by default.

Note that «bar» in a template expression like {{ foo.bar }} will be interpreted as a literal string and not using the value of the variable «bar», if one exists in the template context.

Variable attributes that begin with an underscore may not be accessed as they’re generally considered private.

Filters¶

You can modify variables for display by using filters.

Filters look like this: {{ name|lower }}. This displays the value of the {{ name }} variable after being filtered through the lower filter, which converts text to lowercase. Use a pipe (|) to apply a filter.

Filters can be «chained.» The output of one filter is applied to the next. {{ text|escape|linebreaks }} is a common idiom for escaping text contents, then converting line breaks to <p> tags.

Some filters take arguments. A filter argument looks like this: {{ bio|truncatewords:30 }}. This will display the first 30 words of the bio variable.

Filter arguments that contain spaces must be quoted; for example, to join a list with commas and spaces you’d use {{ list|join:", " }}.

Django provides about sixty built-in template filters. You can read all about them in the built-in filter reference. To give you a taste of what’s available, here are some of the more commonly used template filters:

default

If a variable is false or empty, use given default. Otherwise, use the value of the variable. For example:

{{ value|default:"nothing" }}

If value isn’t provided or is empty, the above will display «nothing».

length

Returns the length of the value. This works for both strings and lists. For example:

{{ value|length }}

If value is ['a', 'b', 'c', 'd'], the output will be 4.

filesizeformat

Formats the value like a «human-readable» file size (i.e. '13 KB', '4.1 MB', '102 bytes', etc.). For example:

{{ value|filesizeformat }}

If value is 123456789, the output would be 117.7 MB.

Again, these are just a few examples; see the built-in filter reference for the complete list.

You can also create your own custom template filters; see Come creare tag di template e filtri personalizzati.

Tags¶

Tags look like this: {% tag %}. Tags are more complex than variables: Some create text in the output, some control flow by performing loops or logic, and some load external information into the template to be used by later variables.

Some tags require beginning and ending tags (i.e. {% tag %} ... tag contents ... {% endtag %}).

Django ships with about two dozen built-in template tags. You can read all about them in the built-in tag reference. To give you a taste of what’s available, here are some of the more commonly used tags:

for

Loop over each item in an array. For example, to display a list of athletes provided in athlete_list:

<ul>
{% for athlete in athlete_list %}
    <li>{{ athlete.name }}</li>
{% endfor %}
</ul>

if, elif, and else

Evaluates a variable, and if that variable is «true» the contents of the block are displayed:

{% if athlete_list %}
    Number of athletes: {{ athlete_list|length }}
{% elif athlete_in_locker_room_list %}
    Athletes should be out of the locker room soon!
{% else %}
    No athletes.
{% endif %}

In the above, if athlete_list is not empty, the number of athletes will be displayed by the {{ athlete_list|length }} variable. Otherwise, if athlete_in_locker_room_list is not empty, the message «Athletes should be out…» will be displayed. If both lists are empty, «No athletes.» will be displayed.

You can also use filters and various operators in the if tag:

{% if athlete_list|length > 1 %}
   Team: {% for athlete in athlete_list %} ... {% endfor %}
{% else %}
   Athlete: {{ athlete_list.0.name }}
{% endif %}

While the above example works, be aware that most template filters return strings, so mathematical comparisons using filters will generally not work as you expect. length is an exception.

block and extends

Set up template inheritance (see below), a powerful way of cutting down on «boilerplate» in templates.

Again, the above is only a selection of the whole list; see the built-in tag reference for the complete list.

You can also create your own custom template tags; see Come creare tag di template e filtri personalizzati.

Comments¶

To comment-out part of a line in a template, use the comment syntax: {# #}.

For example, this template would render as 'hello':

{# greeting #}hello

A comment can contain any template code, invalid or not. For example:

{# {% if foo %}bar{% else %} #}

This syntax can only be used for single-line comments (no newlines are permitted between the {# and #} delimiters). If you need to comment out a multiline portion of the template, see the comment tag.

Template inheritance¶

The most powerful – and thus the most complex – part of Django’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.

Let’s look at template inheritance by starting with an example:

<!DOCTYPE html>
<html lang="en">
<head>
    <link rel="stylesheet" href="style.css">
    <title>{% block title %}My amazing site{% endblock %}</title>
</head>

<body>
    <div id="sidebar">
        {% block sidebar %}
        <ul>
            <li><a href="/">Home</a></li>
            <li><a href="/blog/">Blog</a></li>
        </ul>
        {% endblock %}
    </div>

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

This template, which we’ll call base.html, defines an HTML skeleton document that you might use for a 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.html" %}

{% block title %}My amazing blog{% endblock %}

{% block content %}
{% 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.html».

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">
<head>
    <link rel="stylesheet" href="style.css">
    <title>My amazing blog</title>
</head>

<body>
    <div id="sidebar">
        <ul>
            <li><a href="/">Home</a></li>
            <li><a href="/blog/">Blog</a></li>
        </ul>
    </div>

    <div id="content">
        <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.html template that holds the main look-and-feel of your site.
• Create a base_SECTIONNAME.html template for each «section» of your site. For example, base_news.html, base_sports.html. 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 helps 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.

• By using the same template name as you are inheriting from, {% extends %} can be used to inherit a template at the same time as overriding it. Combined with {{ block.super }}, this can be a powerful way to make small customizations. See Estendere un template sovrascritto in the Overriding templates How-to for a full example.

• 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:

  {% translate "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.

• {% block %} tags are evaluated first. That’s why the content of a block is always overridden, regardless of the truthiness of surrounding tags. For example, this template will always override the content of the title block:

  {% if change_title %}
      {% block title %}Hello!{% endblock title %}
  {% endif %}
  

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.

Automatic HTML escaping¶

When generating HTML from templates, there’s always a risk that a variable will include characters that affect the resulting HTML. For example, consider this template fragment:

Hello, {{ name }}

At first, this seems like a harmless way to display a user’s name, but consider what would happen if the user entered their name as this:

<script>alert('hello')</script>

With this name value, the template would be rendered as:

Hello, <script>alert('hello')</script>

…which means the browser would pop-up a JavaScript alert box!

Similarly, what if the name contained a '<' symbol, like this?

<b>username

That would result in a rendered template like this:

Hello, <b>username

…which, in turn, would result in the remainder of the web page being in bold!

Clearly, user-submitted data shouldn’t be trusted blindly and inserted directly into your web pages, because a malicious user could use this kind of hole to do potentially bad things. This type of security exploit is called a Cross Site Scripting (XSS) attack.

To avoid this problem, you have two options:

• One, you can make sure to run each untrusted variable through the escape filter (documented below), which converts potentially harmful HTML characters to unharmful ones. This was the default solution in Django for its first few years, but the problem is that it puts the onus on you, the developer / template author, to ensure you’re escaping everything. It’s easy to forget to escape data.
• Two, you can take advantage of Django’s automatic HTML escaping. The remainder of this section describes how auto-escaping works.

By default in Django, every template automatically escapes the output of every variable tag. Specifically, these five characters are escaped:

• < is converted to <
• > is converted to >
• ' (single quote) is converted to '
• " (double quote) is converted to "
• & is converted to &

Again, we stress that this behavior is on by default. If you’re using Django’s template system, you’re protected.

This will be escaped: {{ data }}
This will not be escaped: {{ data|safe }}

This will be escaped: <b>
This will not be escaped: <b>

{% autoescape off %}
    Hello {{ name }}
{% endautoescape %}

Auto-escaping is on by default. Hello {{ name }}

{% autoescape off %}
    This will not be auto-escaped: {{ data }}.

    Nor this: {{ other_data }}
    {% autoescape on %}
        Auto-escaping applies again: {{ name }}
    {% endautoescape %}
{% endautoescape %}

{% autoescape off %}
<h1>{% block title %}{% endblock %}</h1>
{% block content %}
{% endblock %}
{% endautoescape %}

{% extends "base.html" %}
{% block title %}This & that{% endblock %}
{% block content %}{{ greeting }}{% endblock %}

<h1>This &amp; that</h1>
<b>Hello!</b>

{{ data|default:"This is a string literal." }}

{{ data|default:"3 < 2" }}

{{ data|default:"3 < 2" }}  {# Bad! Don't do this. #}

Accessing method calls¶

Most method calls attached to objects are also available from within templates. This means that templates have access to much more than just class attributes (like field names) and variables passed in from views. For example, the Django ORM provides the «entry_set» syntax for finding a collection of objects related on a foreign key. Therefore, given a model called «comment» with a foreign key relationship to a model called «task» you can loop through all comments attached to a given task like this:

{% for comment in task.comment_set.all %}
    {{ comment }}
{% endfor %}

Similarly, QuerySets provide a count() method to count the number of objects they contain. Therefore, you can obtain a count of all comments related to the current task with:

{{ task.comment_set.all.count }}

You can also access methods you’ve explicitly defined on your own models:

class Task(models.Model):
    def foo(self):
        return "bar"

{{ task.foo }}

Because Django intentionally limits the amount of logic processing available in the template language, it is not possible to pass arguments to method calls accessed from within templates. Data should be calculated in views, then passed to templates for display.

Custom tag and filter libraries¶

Certain applications provide custom tag and filter libraries. To access them in a template, ensure the application is in INSTALLED_APPS (we’d add 'django.contrib.humanize' for this example), and then use the load tag in a template:

{% load humanize %}

{{ 45000|intcomma }}

In the above, the load tag loads the humanize tag library, which then makes the intcomma filter available for use. If you’ve enabled django.contrib.admindocs, you can consult the documentation area in your admin to find the list of custom libraries in your installation.

The load tag can take multiple library names, separated by spaces. Example:

{% load humanize i18n %}

See Come creare tag di template e filtri personalizzati for information on writing your own custom template libraries.