django/docs/topics/templates.txt

.. _topics-templates:

============================
The Django template language
============================

.. admonition:: 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 :ref:`ref-templates-api`.

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.

.. admonition:: 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 :ttag:`if` tag for boolean tests, a :ttag:`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 :ref:`your own extensions
    <howto-custom-template-tags>` to the template language as needed).

.. _`The Django template language: For Python programmers`: ../templates_python/
.. _Smarty: http://smarty.php.net/
.. _CheetahTemplate: http://www.cheetahtemplate.org/

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 %}

.. admonition:: 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.

.. admonition:: 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 :ref:`built-in filter reference <ref-templates-builtins-filters>`.
To give you a taste of what's available, here are some of the more commonly used
template filters:

    :tfilter:`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``".
        
    :tfilter:`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``.
    
    :tfilter:`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 :ref:`built-in filter reference
<ref-templates-builtins-filters>` for the complete list.

You can also create your own custom template filters; see
:ref:`howto-custom-template-tags`.

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 :ref:`built-in tag reference <ref-templates-builtins-tags>`. To give
you a taste of what's available, here are some of the more commonly used
tags:

    :ttag:`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>
        
    :ttag:`if` and :ttag:`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 }}
            {% 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.
        
    :ttag:`ifequal` and :ttag:`ifnotequal`
        Display some contents if two arguments are or are not equal. For example::

            {% ifequal athlete.name coach.name %}
                ...
            {% endifequal %}

        Or::

            {% ifnotequal athlete.name "Joe" %}
                ...
            {% endifequal %}
    
    :ttag:`block` and :ttag:`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 :ref:`built-in
tag reference <ref-templates-builtins-tags>` for the complete list.

You can also create your own custom template tags; see
:ref:`howto-custom-template-tags`.

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 :ttag:`comment` tag.

.. _template-inheritance:

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 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.

    * 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.

.. _next section: #automatic-html-escaping
.. _automatic-html-escaping:

Automatic HTML escaping
=======================

**New in Django development version**

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 the Django development version, every template automatically
escapes the output of every variable tag. Specifically, these five characters
are escaped:

    * ``<`` is converted to ``&lt;``
    * ``>`` is converted to ``&gt;``
    * ``'`` (single quote) is converted to ``&#39;``
    * ``"`` (double quote) is converted to ``&quot;``
    * ``&`` is converted to ``&amp;``

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

.. _Cross Site Scripting: http://en.wikipedia.org/wiki/Cross-site_scripting

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: &lt;b&gt;
    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 &lt; 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.

.. _template-built-in-reference:

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 :ref:`howto-custom-template-tags` 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.