The Django template language¶
About this document
This document explains the language syntax of the Django template system. If you’re looking for a more technical perspective on how it works and how to extend it, see The Django template language: For Python programmers.
Django’s template language is designed to strike a balance between power and ease. It’s designed to feel comfortable to those used to working with HTML. If you have any exposure to other text-based template languages, such as Smarty or CheetahTemplate, you should feel right at home with Django’s templates.
Philosophy
If you have a background in programming, or if you’re used to languages like PHP which mix programming code directly into HTML, you’ll want to bear in mind that the Django template system is not simply Python embedded into HTML. This is by design: the template system is meant to express presentation, not program logic.
The Django template system provides tags which function similarly to some
programming constructs – an if
tag for boolean tests, a for
tag for looping, etc. – but these are not simply executed as the
corresponding Python code, and the template system will not execute
arbitrary Python expressions. Only the tags, filters and syntax listed below
are supported by default (although you can add your own extensions to the template language as needed).
Templates¶
A template is simply 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 %}
Philosophy
Why use a text-based template instead of an XML-based one (like Zope’s TAL)? We wanted Django’s template language to be usable for more than just XML/HTML templates. At World Online, we use it for e-mails, JavaScript and CSV. You can use the template language for any text-based format.
Oh, and one more thing: Making humans edit XML is sadistic!
Variables¶
Variables look like this: {{ variable }}
. When the template engine
encounters a variable, it evaluates that variable and replaces it with the
result.
Use a dot (.
) to access attributes of a variable.
Behind the scenes
Technically, when the template system encounters a dot, it tries the following lookups, in this order:
- Dictionary lookup
- Attribute lookup
- Method call
- List-index lookup
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 TEMPLATE_STRING_IF_INVALID
setting, which is set to ''
(the empty string) by default.
See Using the built-in reference, below, for help on finding what variables are available in a given template.
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 spaced you’d use {{ list|join:", " }}
.
Django provides about thirty 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 be4
.striptags
Strips all [X]HTML tags. For example:
{{ value|striptags }}
If
value
is"<b>Joel</b> <button>is</button> a <span>slug</span>"
, the output will be"Joel is a slug"
.
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 Tags e filtros de template personalizados.
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
andelse
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 }} {% 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.ifequal
andifnotequal
Display some contents if two arguments are or are not equal. For example:
{% ifequal athlete.name coach.name %} ... {% endifequal %}
Or:
{% ifnotequal athlete.name "Joe" %} ... {% endifnotequal %}
block
andextends
- 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 Tags e filtros de template personalizados.
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.
It’s easiest to understand template inheritance by starting with an example:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" 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 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.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 PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" 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 extendbase.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.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.
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 his 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 bolded!
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.
How to turn it off¶
If you don’t want data to be auto-escaped, on a per-site, per-template level or per-variable level, you can turn it off in several ways.
Why would you want to turn it off? Because sometimes, template variables contain data that you intend to be rendered as raw HTML, in which case you don’t want their contents to be escaped. For example, you might store a blob of HTML in your database and want to embed that directly into your template. Or, you might be using Django’s template system to produce text that is not HTML – like an e-mail message, for instance.
For individual variables¶
To disable auto-escaping for an individual variable, use the safe
filter:
This will be escaped: {{ data }}
This will not be escaped: {{ data|safe }}
Think of safe as shorthand for safe from further escaping or can be
safely interpreted as HTML. In this example, if data
contains '<b>'
,
the output will be:
This will be escaped: <b>
This will not be escaped: <b>
For template blocks¶
To control auto-escaping for a template, wrap the template (or just a
particular section of the template) in the autoescape
tag, like so:
{% autoescape off %}
Hello {{ name }}
{% endautoescape %}
The autoescape
tag takes either on
or off
as its argument. At
times, you might want to force auto-escaping when it would otherwise be
disabled. Here is an example template:
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 %}
The auto-escaping tag passes its effect onto templates that extend the
current one as well as templates included via the include
tag, just like
all block tags. For example:
# base.html
{% autoescape off %}
<h1>{% block title %}{% endblock %}</h1>
{% block content %}
{% endblock %}
{% endautoescape %}
# child.html
{% extends "base.html" %}
{% block title %}This & that{% endblock %}
{% block content %}{{ greeting }}{% endblock %}
Because auto-escaping is turned off in the base template, it will also be
turned off in the child template, resulting in the following rendered
HTML when the greeting
variable contains the string <b>Hello!</b>
:
<h1>This & that</h1>
<b>Hello!</b>
Notes¶
Generally, template authors don’t need to worry about auto-escaping very much. Developers on the Python side (people writing views and custom filters) need to think about the cases in which data shouldn’t be escaped, and mark data appropriately, so things Just Work in the template.
If you’re creating a template that might be used in situations where you’re
not sure whether auto-escaping is enabled, then add an escape
filter to any
variable that needs escaping. When auto-escaping is on, there’s no danger of
the escape
filter double-escaping data – the escape
filter does not
affect auto-escaped variables.
String literals and automatic escaping¶
As we mentioned earlier, filter arguments can be strings:
{{ data|default:"This is a string literal." }}
All string literals are inserted without any automatic escaping into the
template – they act as if they were all passed through the safe
filter.
The reasoning behind this is that the template author is in control of what
goes into the string literal, so they can make sure the text is correctly
escaped when the template is written.
This means you would write
{{ data|default:"3 < 2" }}
...rather than
{{ data|default:"3 < 2" }} <-- Bad! Don't do this.
This doesn’t affect what happens to data coming from the variable itself. The variable’s contents are still automatically escaped, if necessary, because they’re beyond the control of the template author.
Using the built-in reference¶
Django’s admin interface includes a complete reference of all template tags and filters available for a given site. To see it, go to your admin interface and click the “Documentation” link in the upper right of the page.
The reference is divided into 4 sections: tags, filters, models, and views.
The tags and filters sections describe all the built-in tags (in fact, the tag and filter references below come directly from those pages) as well as any custom tag or filter libraries available.
The views page is the most valuable. Each URL in your site has a separate entry here, and clicking on a URL will show you:
- The name of the view function that generates that view.
- A short description of what the view does.
- The context, or a list of variables available in the view’s template.
- The name of the template or templates that are used for that view.
Each view documentation page also has a bookmarklet that you can use to jump from any page to the documentation page for that view.
Because Django-powered sites usually use database objects, the models section of the documentation page describes each type of object in the system along with all the fields available on that object.
Taken together, the documentation pages should tell you every tag, filter, variable and object available to you in a given template.
Custom tag and filter libraries¶
Certain applications provide custom tag and filter libraries. To access them in
a template, use the {% load %}
tag:
{% load comments %}
{% comment_form for blogs.entries entry.id with is_public yes %}
In the above, the load
tag loads the comments
tag library, which then
makes the comment_form
tag available for use. 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 comments i18n %}
See Tags e filtros de template personalizados for information on writing your own custom template libraries.
Custom libraries and template inheritance¶
When you load a custom tag or filter library, the tags/filters are only made available to the current template – not any parent or child templates along the template-inheritance path.
For example, if a template foo.html
has {% load comments %}
, a child
template (e.g., one that has {% extends "foo.html" %}
) will not have
access to the comments template tags and filters. The child template is
responsible for its own {% load comments %}
.
This is a feature for the sake of maintainability and sanity.