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682 lines
24 KiB
Plaintext
============================
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The Django template language
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============================
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.. admonition:: About this document
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This document explains the language syntax of the Django template system. If
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you're looking for a more technical perspective on how it works and how to
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extend it, see :doc:`/ref/templates/api`.
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Django's template language is designed to strike a balance between power and
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ease. It's designed to feel comfortable to those used to working with HTML. If
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you have any exposure to other text-based template languages, such as Smarty_
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or CheetahTemplate_, you should feel right at home with Django's templates.
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.. admonition:: Philosophy
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If you have a background in programming, or if you're used to languages
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like PHP which mix programming code directly into HTML, you'll want to
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bear in mind that the Django template system is not simply Python embedded
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into HTML. This is by design: the template system is meant to express
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presentation, not program logic.
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The Django template system provides tags which function similarly to some
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programming constructs -- an :ttag:`if` tag for boolean tests, a :ttag:`for`
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tag for looping, etc. -- but these are not simply executed as the
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corresponding Python code, and the template system will not execute
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arbitrary Python expressions. Only the tags, filters and syntax listed below
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are supported by default (although you can add :doc:`your own extensions
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</howto/custom-template-tags>` to the template language as needed).
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.. _`The Django template language: For Python programmers`: ../templates_python/
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.. _Smarty: http://smarty.php.net/
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.. _CheetahTemplate: http://www.cheetahtemplate.org/
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Templates
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=========
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.. highlightlang:: html+django
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A template is simply a text file. It can generate any text-based format (HTML,
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XML, CSV, etc.).
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A template contains **variables**, which get replaced with values when the
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template is evaluated, and **tags**, which control the logic of the template.
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Below is a minimal template that illustrates a few basics. Each element will be
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explained later in this document.::
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{% extends "base_generic.html" %}
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{% block title %}{{ section.title }}{% endblock %}
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{% block content %}
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<h1>{{ section.title }}</h1>
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{% for story in story_list %}
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<h2>
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<a href="{{ story.get_absolute_url }}">
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{{ story.headline|upper }}
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</a>
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</h2>
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<p>{{ story.tease|truncatewords:"100" }}</p>
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{% endfor %}
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{% endblock %}
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.. admonition:: Philosophy
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Why use a text-based template instead of an XML-based one (like Zope's
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TAL)? We wanted Django's template language to be usable for more than
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just XML/HTML templates. At World Online, we use it for emails,
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JavaScript and CSV. You can use the template language for any text-based
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format.
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Oh, and one more thing: Making humans edit XML is sadistic!
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Variables
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=========
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Variables look like this: ``{{ variable }}``. When the template engine
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encounters a variable, it evaluates that variable and replaces it with the
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result. Variable names consist of any combination of alphanumeric characters
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and the underscore (``"_"``). The dot (``"."``) also appears in variable
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sections, although that has a special meaning, as indicated below.
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Importantly, *you cannot have spaces or punctuation characters in variable
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names.*
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Use a dot (``.``) to access attributes of a variable.
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.. admonition:: Behind the scenes
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Technically, when the template system encounters a dot, it tries the
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following lookups, in this order:
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* Dictionary lookup
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* Attribute lookup
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* Method call
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* List-index lookup
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This can cause some unexpected behavior with objects that override
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dictionary lookup. For example, consider the following code snippet that
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attempts to loop over a ``collections.defaultdict``::
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{% for k, v in defaultdict.iteritems %}
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Do something with k and v here...
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{% endfor %}
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Because dictionary lookup happens first, that behavior kicks in and provides
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a default value instead of using the intended ``.iteritems()``
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method. In this case, consider converting to a dictionary first.
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In the above example, ``{{ section.title }}`` will be replaced with the
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``title`` attribute of the ``section`` object.
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If you use a variable that doesn't exist, the template system will insert
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the value of the :setting:`TEMPLATE_STRING_IF_INVALID` setting, which is set
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to ``''`` (the empty string) by default.
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Note that "bar" in a template expression like ``{{ foo.bar }}`` will be
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interpreted as a literal string and not using the value of the variable "bar",
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if one exists in the template context.
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Filters
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=======
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You can modify variables for display by using **filters**.
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Filters look like this: ``{{ name|lower }}``. This displays the value of the
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``{{ name }}`` variable after being filtered through the :tfilter:`lower`
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filter, which converts text to lowercase. Use a pipe (``|``) to apply a filter.
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Filters can be "chained." The output of one filter is applied to the next.
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``{{ text|escape|linebreaks }}`` is a common idiom for escaping text contents,
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then converting line breaks to ``<p>`` tags.
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Some filters take arguments. A filter argument looks like this: ``{{
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bio|truncatewords:30 }}``. This will display the first 30 words of the ``bio``
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variable.
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Filter arguments that contain spaces must be quoted; for example, to join a
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list with commas and spaced you'd use ``{{ list|join:", " }}``.
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Django provides about thirty built-in template filters. You can read all about
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them in the :ref:`built-in filter reference <ref-templates-builtins-filters>`.
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To give you a taste of what's available, here are some of the more commonly
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used template filters:
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:tfilter:`default`
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If a variable is false or empty, use given default. Otherwise, use the
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value of the variable
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For example::
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{{ value|default:"nothing" }}
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If ``value`` isn't provided or is empty, the above will display
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"``nothing``".
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:tfilter:`length`
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Returns the length of the value. This works for both strings and lists;
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for example::
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{{ value|length }}
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If ``value`` is ``['a', 'b', 'c', 'd']``, the output will be ``4``.
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:tfilter:`striptags`
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Strips all [X]HTML tags. For example::
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{{ value|striptags }}
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If ``value`` is ``"<b>Joel</b> <button>is</button> a
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<span>slug</span>"``, the output will be ``"Joel is a slug"``.
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Again, these are just a few examples; see the :ref:`built-in filter reference
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<ref-templates-builtins-filters>` for the complete list.
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You can also create your own custom template filters; see
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:doc:`/howto/custom-template-tags`.
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.. seealso::
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Django's admin interface can include a complete reference of all template
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tags and filters available for a given site. See
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:doc:`/ref/contrib/admin/admindocs`.
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Tags
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====
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Tags look like this: ``{% tag %}``. Tags are more complex than variables: Some
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create text in the output, some control flow by performing loops or logic, and
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some load external information into the template to be used by later variables.
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Some tags require beginning and ending tags (i.e. ``{% tag %} ... tag contents
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... {% endtag %}``).
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Django ships with about two dozen built-in template tags. You can read all about
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them in the :ref:`built-in tag reference <ref-templates-builtins-tags>`. To give
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you a taste of what's available, here are some of the more commonly used
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tags:
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:ttag:`for`
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Loop over each item in an array. For example, to display a list of athletes
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provided in ``athlete_list``::
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<ul>
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{% for athlete in athlete_list %}
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<li>{{ athlete.name }}</li>
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{% endfor %}
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</ul>
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:ttag:`if` and ``else``
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Evaluates a variable, and if that variable is "true" the contents of the
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block are displayed::
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{% if athlete_list %}
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Number of athletes: {{ athlete_list|length }}
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{% else %}
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No athletes.
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{% endif %}
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In the above, if ``athlete_list`` is not empty, the number of athletes
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will be displayed by the ``{{ athlete_list|length }}`` variable.
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You can also use filters and various operators in the :ttag:`if` tag::
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{% if athlete_list|length > 1 %}
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Team: {% for athlete in athlete_list %} ... {% endfor %}
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{% else %}
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Athlete: {{ athlete_list.0.name }}
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{% endif %}
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While the above example works, be aware that most template filters return
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strings, so mathematical comparisons using filters will generally not work
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as you expect. :tfilter:`length` is an exception.
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:ttag:`block` and :ttag:`extends`
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Set up `template inheritance`_ (see below), a powerful way
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of cutting down on "boilerplate" in templates.
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Again, the above is only a selection of the whole list; see the :ref:`built-in
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tag reference <ref-templates-builtins-tags>` for the complete list.
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You can also create your own custom template tags; see
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:doc:`/howto/custom-template-tags`.
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.. seealso::
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Django's admin interface can include a complete reference of all template
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tags and filters available for a given site. See
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:doc:`/ref/contrib/admin/admindocs`.
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Comments
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========
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To comment-out part of a line in a template, use the comment syntax: ``{# #}``.
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For example, this template would render as ``'hello'``::
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{# greeting #}hello
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A comment can contain any template code, invalid or not. For example::
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{# {% if foo %}bar{% else %} #}
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This syntax can only be used for single-line comments (no newlines are permitted
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between the ``{#`` and ``#}`` delimiters). If you need to comment out a
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multiline portion of the template, see the :ttag:`comment` tag.
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.. _template-inheritance:
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Template inheritance
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====================
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The most powerful -- and thus the most complex -- part of Django's template
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engine is template inheritance. Template inheritance allows you to build a base
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"skeleton" template that contains all the common elements of your site and
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defines **blocks** that child templates can override.
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It's easiest to understand template inheritance by starting with an example::
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<link rel="stylesheet" href="style.css" />
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<title>{% block title %}My amazing site{% endblock %}</title>
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</head>
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<body>
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<div id="sidebar">
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{% block sidebar %}
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<ul>
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<li><a href="/">Home</a></li>
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<li><a href="/blog/">Blog</a></li>
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</ul>
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{% endblock %}
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</div>
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<div id="content">
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{% block content %}{% endblock %}
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</div>
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</body>
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</html>
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This template, which we'll call ``base.html``, defines a simple HTML skeleton
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document that you might use for a simple two-column page. It's the job of
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"child" templates to fill the empty blocks with content.
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In this example, the :ttag:`block` tag defines three blocks that child
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templates can fill in. All the :ttag:`block` tag does is to tell the template
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engine that a child template may override those portions of the template.
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A child template might look like this::
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{% extends "base.html" %}
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{% block title %}My amazing blog{% endblock %}
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{% block content %}
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{% for entry in blog_entries %}
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<h2>{{ entry.title }}</h2>
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<p>{{ entry.body }}</p>
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{% endfor %}
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{% endblock %}
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The :ttag:`extends` tag is the key here. It tells the template engine that
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this template "extends" another template. When the template system evaluates
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this template, first it locates the parent -- in this case, "base.html".
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At that point, the template engine will notice the three :ttag:`block` tags
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in ``base.html`` and replace those blocks with the contents of the child
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template. Depending on the value of ``blog_entries``, the output might look
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like::
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<link rel="stylesheet" href="style.css" />
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<title>My amazing blog</title>
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</head>
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<body>
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<div id="sidebar">
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<ul>
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<li><a href="/">Home</a></li>
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<li><a href="/blog/">Blog</a></li>
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</ul>
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</div>
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<div id="content">
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<h2>Entry one</h2>
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<p>This is my first entry.</p>
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<h2>Entry two</h2>
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<p>This is my second entry.</p>
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</div>
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</body>
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</html>
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Note that since the child template didn't define the ``sidebar`` block, the
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value from the parent template is used instead. Content within a ``{% block %}``
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tag in a parent template is always used as a fallback.
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You can use as many levels of inheritance as needed. One common way of using
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inheritance is the following three-level approach:
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* Create a ``base.html`` template that holds the main look-and-feel of your
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site.
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* Create a ``base_SECTIONNAME.html`` template for each "section" of your
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site. For example, ``base_news.html``, ``base_sports.html``. These
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templates all extend ``base.html`` and include section-specific
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styles/design.
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* Create individual templates for each type of page, such as a news
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article or blog entry. These templates extend the appropriate section
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template.
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This approach maximizes code reuse and makes it easy to add items to shared
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content areas, such as section-wide navigation.
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Here are some tips for working with inheritance:
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* If you use :ttag:`{% extends %}<extends>` in a template, it must be the first template
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tag in that template. Template inheritance won't work, otherwise.
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* More :ttag:`{% block %}<block>` tags in your base templates are better. Remember,
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child templates don't have to define all parent blocks, so you can fill
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in reasonable defaults in a number of blocks, then only define the ones
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you need later. It's better to have more hooks than fewer hooks.
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* If you find yourself duplicating content in a number of templates, it
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probably means you should move that content to a ``{% block %}`` in a
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parent template.
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* If you need to get the content of the block from the parent template,
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the ``{{ block.super }}`` variable will do the trick. This is useful
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if you want to add to the contents of a parent block instead of
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completely overriding it. Data inserted using ``{{ block.super }}`` will
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not be automatically escaped (see the `next section`_), since it was
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already escaped, if necessary, in the parent template.
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* For extra readability, you can optionally give a *name* to your
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``{% endblock %}`` tag. For example::
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{% block content %}
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...
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{% endblock content %}
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In larger templates, this technique helps you see which ``{% block %}``
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tags are being closed.
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Finally, note that you can't define multiple :ttag:`block` tags with the same
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name in the same template. This limitation exists because a block tag works in
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"both" directions. That is, a block tag doesn't just provide a hole to fill --
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it also defines the content that fills the hole in the *parent*. If there were
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two similarly-named :ttag:`block` tags in a template, that template's parent
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wouldn't know which one of the blocks' content to use.
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.. _next section: #automatic-html-escaping
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.. _automatic-html-escaping:
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Automatic HTML escaping
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=======================
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When generating HTML from templates, there's always a risk that a variable will
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include characters that affect the resulting HTML. For example, consider this
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template fragment::
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Hello, {{ name }}.
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At first, this seems like a harmless way to display a user's name, but consider
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what would happen if the user entered his name as this::
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<script>alert('hello')</script>
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With this name value, the template would be rendered as::
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Hello, <script>alert('hello')</script>
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...which means the browser would pop-up a JavaScript alert box!
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Similarly, what if the name contained a ``'<'`` symbol, like this?
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.. code-block:: html
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<b>username
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That would result in a rendered template like this::
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Hello, <b>username
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...which, in turn, would result in the remainder of the Web page being bolded!
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Clearly, user-submitted data shouldn't be trusted blindly and inserted directly
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into your Web pages, because a malicious user could use this kind of hole to
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do potentially bad things. This type of security exploit is called a
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`Cross Site Scripting`_ (XSS) attack.
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To avoid this problem, you have two options:
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* One, you can make sure to run each untrusted variable through the
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:tfilter:`escape` filter (documented below), which converts potentially
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harmful HTML characters to unharmful ones. This was the default solution
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in Django for its first few years, but the problem is that it puts the
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onus on *you*, the developer / template author, to ensure you're escaping
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everything. It's easy to forget to escape data.
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* Two, you can take advantage of Django's automatic HTML escaping. The
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remainder of this section describes how auto-escaping works.
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By default in Django, every template automatically escapes the output
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of every variable tag. Specifically, these five characters are
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escaped:
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* ``<`` is converted to ``<``
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* ``>`` is converted to ``>``
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* ``'`` (single quote) is converted to ``'``
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* ``"`` (double quote) is converted to ``"``
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* ``&`` is converted to ``&``
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Again, we stress that this behavior is on by default. If you're using Django's
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template system, you're protected.
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.. _Cross Site Scripting: http://en.wikipedia.org/wiki/Cross-site_scripting
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How to turn it off
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------------------
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If you don't want data to be auto-escaped, on a per-site, per-template level or
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per-variable level, you can turn it off in several ways.
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Why would you want to turn it off? Because sometimes, template variables
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contain data that you *intend* to be rendered as raw HTML, in which case you
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don't want their contents to be escaped. For example, you might store a blob of
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HTML in your database and want to embed that directly into your template. Or,
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you might be using Django's template system to produce text that is *not* HTML
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-- like an email message, for instance.
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For individual variables
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~~~~~~~~~~~~~~~~~~~~~~~~
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To disable auto-escaping for an individual variable, use the :tfilter:`safe`
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filter::
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This will be escaped: {{ data }}
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This will not be escaped: {{ data|safe }}
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Think of *safe* as shorthand for *safe from further escaping* or *can be
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safely interpreted as HTML*. In this example, if ``data`` contains ``'<b>'``,
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the output will be::
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This will be escaped: <b>
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This will not be escaped: <b>
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For template blocks
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~~~~~~~~~~~~~~~~~~~
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To control auto-escaping for a template, wrap the template (or just a
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particular section of the template) in the :ttag:`autoescape` tag, like so::
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{% autoescape off %}
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Hello {{ name }}
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|
{% endautoescape %}
|
|
|
|
The :ttag:`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 :ttag:`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 :tfilter:`escape` filter
|
|
to any variable that needs escaping. When auto-escaping is on, there's no
|
|
danger of the :tfilter:`escape` filter *double-escaping* data -- the
|
|
:tfilter:`escape` filter does not affect auto-escaped variables.
|
|
|
|
.. _string-literals-and-automatic-escaping:
|
|
|
|
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 :tfilter:`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.
|
|
|
|
.. _template-accessing-methods:
|
|
|
|
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 :ref:`"entry_set"<topics-db-queries-related>` 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, :doc:`QuerySets</ref/models/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 }}
|
|
|
|
And of course you can easily access methods you've explicitly defined on your
|
|
own models::
|
|
|
|
# In model
|
|
class Task(models.Model):
|
|
def foo(self):
|
|
return "bar"
|
|
|
|
# In template
|
|
{{ 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.
|
|
|
|
.. _loading-custom-template-libraries:
|
|
|
|
Custom tag and filter libraries
|
|
===============================
|
|
|
|
Certain applications provide custom tag and filter libraries. To access them in
|
|
a template, use the :ttag:`load` tag::
|
|
|
|
{% load comments %}
|
|
|
|
{% comment_form for blogs.entries entry.id with is_public yes %}
|
|
|
|
In the above, the :ttag:`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 :ttag:`load` tag can take multiple library names, separated by spaces.
|
|
Example::
|
|
|
|
{% load comments i18n %}
|
|
|
|
See :doc:`/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.
|