django/docs/topics/templates.txt

=========
Templates
=========

.. module:: django.template
    :synopsis: Django's template system

Being a web framework, Django needs a convenient way to generate HTML
dynamically. The most common approach relies on templates. A template contains
the static parts of the desired HTML output as well as some special syntax
describing how dynamic content will be inserted. For a hands-on example of
creating HTML pages with templates, see :doc:`Tutorial 3 </intro/tutorial03>`.

A Django project can be configured with one or several template engines (or
even zero if you don't use templates). Django ships built-in backends for its
own template system, creatively called the Django template language (DTL), and
for the popular alternative Jinja2_. Backends for other template languages may
be available from third-parties.

Django defines a standard API for loading and rendering templates regardless
of the backend. Loading consists of finding the template for a given identifier
and preprocessing it, usually compiling it to an in-memory representation.
Rendering means interpolating the template with context data and returning the
resulting string.

The :doc:`Django template language </ref/templates/language>` is Django's own
template system. Until Django 1.8 it was the only built-in option available.
It's a good template library even though it's fairly opinionated and sports a
few idiosyncrasies. If you don't have a pressing reason to choose another
backend, you should use the DTL, especially if you're writing a pluggable
application and you intend to distribute templates. Django's contrib apps that
include templates, like :doc:`django.contrib.admin </ref/contrib/admin/index>`,
use the DTL.

For historical reasons, both the generic support for template engines and the
implementation of the Django template language live in the ``django.template``
namespace.

.. _template-engines:

Support for template engines
============================

Configuration
-------------

Templates engines are configured with the :setting:`TEMPLATES` setting. It's a
list of configurations, one for each engine. The default value is empty. The
``settings.py`` generated by the :djadmin:`startproject` command defines a
more useful value::

    TEMPLATES = [
        {
            'BACKEND': 'django.template.backends.django.DjangoTemplates',
            'DIRS': [],
            'APP_DIRS': True,
            'OPTIONS': {
                # ... some options here ...
            },
        },
    ]

:setting:`BACKEND <TEMPLATES-BACKEND>` is a dotted Python path to a template
engine class implementing Django's template backend API. The built-in backends
are :class:`django.template.backends.django.DjangoTemplates` and
:class:`django.template.backends.jinja2.Jinja2`.

Since most engines load templates from files, the top-level configuration for
each engine contains two common settings:

* :setting:`DIRS <TEMPLATES-DIRS>` defines a list of directories where the
  engine should look for template source files, in search order.
* :setting:`APP_DIRS <TEMPLATES-APP_DIRS>` tells whether the engine should
  look for templates inside installed applications. Each backend defines a
  conventional name for the subdirectory inside applications where its
  templates should be stored.

While uncommon, it's possible to configure several instances of the same
backend with different options. In that case you should define a unique
:setting:`NAME <TEMPLATES-NAME>` for each engine.

:setting:`OPTIONS <TEMPLATES-OPTIONS>` contains backend-specific settings.

Usage
-----

.. _template-loading:

.. module:: django.template.loader

The ``django.template.loader`` module defines two functions to load templates.

.. function:: get_template(template_name, using=None)

    This function loads the template with the given name and returns a
    ``Template`` object.

    The exact type of the return value depends on the backend that loaded the
    template. Each backend has its own ``Template`` class.

    ``get_template()`` tries each template engine in order until one succeeds.
    If the template cannot be found, it raises
    :exc:`~django.template.TemplateDoesNotExist`. If the template is found but
    contains invalid syntax, it raises
    :exc:`~django.template.TemplateSyntaxError`.

    How templates are searched and loaded depends on each engine's backend and
    configuration.

    If you want to restrict the search to a particular template engine, pass
    the engine's :setting:`NAME <TEMPLATES-NAME>` in the ``using`` argument.

.. function:: select_template(template_name_list, using=None)

    ``select_template()`` is just like ``get_template()``, except it takes a
    list of template names. It tries each name in order and returns the first
    template that exists.

.. currentmodule:: django.template

If loading a template fails, the following two exceptions, defined in
``django.template``, may be raised:

.. exception:: TemplateDoesNotExist(msg, tried=None, backend=None, chain=None)

    This exception is raised when a template cannot be found. It accepts the
    following optional arguments for populating the :ref:`template postmortem
    <template-postmortem>` on the debug page:

    ``backend``
        The template backend instance from which the exception originated.

    ``tried``
        A list of sources that were tried when finding the template. This is
        formatted as a list of tuples containing ``(origin, status)``, where
        ``origin`` is an :ref:`origin-like <template-origin-api>` object and
        ``status`` is a string with the reason the template wasn't found.

    ``chain``
        A list of intermediate :exc:`~django.template.TemplateDoesNotExist`
        exceptions raised when trying to load a template. This is used by
        functions, such as :func:`~django.template.loader.get_template`, that
        try to load a given template from multiple engines.

.. exception:: TemplateSyntaxError(msg)

    This exception is raised when a template was found but contains errors.

``Template`` objects returned by ``get_template()`` and ``select_template()``
must provide a ``render()`` method with the following signature:

.. currentmodule:: django.template.backends.base

.. method:: Template.render(context=None, request=None)

    Renders this template with a given context.

    If ``context`` is provided, it must be a :class:`dict`. If it isn't
    provided, the engine will render the template with an empty context.

    If ``request`` is provided, it must be an :class:`~django.http.HttpRequest`.
    Then the engine must make it, as well as the CSRF token, available in the
    template. How this is achieved is up to each backend.

Here's an example of the search algorithm. For this example the
:setting:`TEMPLATES` setting is::

    TEMPLATES = [
        {
            'BACKEND': 'django.template.backends.django.DjangoTemplates',
            'DIRS': [
                '/home/html/example.com',
                '/home/html/default',
            ],
        },
        {
            'BACKEND': 'django.template.backends.jinja2.Jinja2',
            'DIRS': [
                '/home/html/jinja2',
            ],
        },
    ]

If you call ``get_template('story_detail.html')``, here are the files Django
will look for, in order:

* ``/home/html/example.com/story_detail.html`` (``'django'`` engine)
* ``/home/html/default/story_detail.html`` (``'django'`` engine)
* ``/home/html/jinja2/story_detail.html`` (``'jinja2'`` engine)

If you call ``select_template(['story_253_detail.html', 'story_detail.html'])``,
here's what Django will look for:

* ``/home/html/example.com/story_253_detail.html`` (``'django'`` engine)
* ``/home/html/default/story_253_detail.html`` (``'django'`` engine)
* ``/home/html/jinja2/story_253_detail.html`` (``'jinja2'`` engine)
* ``/home/html/example.com/story_detail.html`` (``'django'`` engine)
* ``/home/html/default/story_detail.html`` (``'django'`` engine)
* ``/home/html/jinja2/story_detail.html`` (``'jinja2'`` engine)

When Django finds a template that exists, it stops looking.

.. admonition:: Tip

    You can use :func:`~django.template.loader.select_template()` for flexible
    template loading. For example, if you've written a news story and want
    some stories to have custom templates, use something like
    ``select_template(['story_%s_detail.html' % story.id,
    'story_detail.html'])``. That'll allow you to use a custom template for an
    individual story, with a fallback template for stories that don't have
    custom templates.

It's possible -- and preferable -- to organize templates in subdirectories
inside each directory containing templates. The convention is to make a
subdirectory for each Django app, with subdirectories within those
subdirectories as needed.

Do this for your own sanity. Storing all templates in the root level of a
single directory gets messy.

To load a template that's within a subdirectory, just use a slash, like so::

    get_template('news/story_detail.html')

Using the same :setting:`TEMPLATES` option as above, this will attempt to load
the following templates:

* ``/home/html/example.com/news/story_detail.html`` (``'django'`` engine)
* ``/home/html/default/news/story_detail.html`` (``'django'`` engine)
* ``/home/html/jinja2/news/story_detail.html`` (``'jinja2'`` engine)

.. currentmodule:: django.template.loader

In addition, to cut down on the repetitive nature of loading and rendering
templates, Django provides a shortcut function which automates the process.

.. function:: render_to_string(template_name, context=None, request=None, using=None)

    ``render_to_string()`` loads a template like :func:`get_template` and
    calls its ``render()`` method immediately. It takes the following
    arguments.

    ``template_name``
        The name of the template to load and render. If it's a list of template
        names, Django uses :func:`select_template` instead of
        :func:`get_template` to find the template.

    ``context``
        A :class:`dict` to be used as the template's context for rendering.

    ``request``
        An optional :class:`~django.http.HttpRequest` that will be available
        during the template's rendering process.

    Usage example::

        from django.template.loader import render_to_string
        rendered = render_to_string('my_template.html', {'foo': 'bar'})

See also the :func:`~django.shortcuts.render()` shortcut which calls
:func:`render_to_string()` and feeds the result into an
:class:`~django.http.HttpResponse` suitable for returning from a view.

Finally, you can use configured engines directly:

.. data:: engines

    Template engines are available in ``django.template.engines``::

        from django.template import engines

        django_engine = engines['django']
        template = django_engine.from_string("Hello {{ name }}!")

    The lookup key — ``'django'`` in this example — is the engine's
    :setting:`NAME <TEMPLATES-NAME>`.

.. module:: django.template.backends

Built-in backends
-----------------

.. module:: django.template.backends.django

.. class:: DjangoTemplates

Set :setting:`BACKEND <TEMPLATES-BACKEND>` to
``'django.template.backends.django.DjangoTemplates'`` to configure a Django
template engine.

When :setting:`APP_DIRS <TEMPLATES-APP_DIRS>` is ``True``, ``DjangoTemplates``
engines look for templates in the ``templates`` subdirectory of installed
applications. This generic name was kept for backwards-compatibility.

``DjangoTemplates`` engines accept the following :setting:`OPTIONS
<TEMPLATES-OPTIONS>`:

* ``'autoescape'``: a boolean that controls whether HTML autoescaping is
  enabled.

  It defaults to ``True``.

  .. warning::

      Only set it to ``False`` if you're rendering non-HTML templates!

  .. versionadded:: 1.10

      The ``autoescape`` option was added.

* ``'context_processors'``: a list of dotted Python paths to callables that
  are used to populate the context when a template is rendered with a request.
  These callables take a request object as their argument and return a
  :class:`dict` of items to be merged into the context.

  It defaults to an empty list.

  See :class:`~django.template.RequestContext` for more information.

* ``'debug'``: a boolean that turns on/off template debug mode. If it is
  ``True``, the fancy error page will display a detailed report for any
  exception raised during template rendering. This report contains the
  relevant snippet of the template with the appropriate line highlighted.

  It defaults to the value of the :setting:`DEBUG` setting.

* ``'loaders'``: a list of dotted Python paths to template loader classes.
  Each ``Loader`` class knows how to import templates from a particular
  source. Optionally, a tuple can be used instead of a string. The first item
  in the tuple should be the ``Loader`` class name, and subsequent items are
  passed to the ``Loader`` during initialization.

  The default depends on the values of :setting:`DIRS <TEMPLATES-DIRS>` and
  :setting:`APP_DIRS <TEMPLATES-APP_DIRS>`.

  See :ref:`template-loaders` for details.

* ``'string_if_invalid'``: the output, as a string, that the template system
  should use for invalid (e.g. misspelled) variables.

  It defaults to an empty string.

  See :ref:`invalid-template-variables` for details.

* ``'file_charset'``: the charset used to read template files on disk.

  It defaults to the value of :setting:`FILE_CHARSET`.

* ``'libraries'``: A dictionary of labels and dotted Python paths of template
  tag modules to register with the template engine. This can be used to add
  new libraries or provide alternate labels for existing ones. For example::

      OPTIONS={
          'libraries': {
              'myapp_tags': 'path.to.myapp.tags',
              'admin.urls': 'django.contrib.admin.templatetags.admin_urls',
          },
      }

  Libraries can be loaded by passing the corresponding dictionary key to
  the :ttag:`{% load %}<load>` tag.

* ``'builtins'``: A list of dotted Python paths of template tag modules to
  add to :doc:`built-ins </ref/templates/builtins>`. For example::

      OPTIONS={
          'builtins': ['myapp.builtins'],
      }

  Tags and filters from built-in libraries can be used without first calling
  the :ttag:`{% load %} <load>` tag.

.. module:: django.template.backends.jinja2

.. class:: Jinja2

Requires Jinja2_ to be installed:

.. code-block:: console

    $ pip install Jinja2

Set :setting:`BACKEND <TEMPLATES-BACKEND>` to
``'django.template.backends.jinja2.Jinja2'`` to configure a Jinja2_ engine.

When :setting:`APP_DIRS <TEMPLATES-APP_DIRS>` is ``True``, ``Jinja2`` engines
look for templates in the ``jinja2`` subdirectory of installed applications.

The most important entry in :setting:`OPTIONS <TEMPLATES-OPTIONS>` is
``'environment'``. It's a dotted Python path to a callable returning a Jinja2
environment. It defaults to ``'jinja2.Environment'``. Django invokes that
callable and passes other options as keyword arguments. Furthermore, Django
adds defaults that differ from Jinja2's for a few options:

* ``'autoescape'``: ``True``
* ``'loader'``: a loader configured for :setting:`DIRS <TEMPLATES-DIRS>` and
  :setting:`APP_DIRS <TEMPLATES-APP_DIRS>`
* ``'auto_reload'``: ``settings.DEBUG``
* ``'undefined'``: ``DebugUndefined if settings.DEBUG else Undefined``

The default configuration is purposefully kept to a minimum. If a template is
rendered with a request (e.g. when using :py:func:`~django.shortcuts.render`),
the ``Jinja2`` backend adds the globals ``request``, ``csrf_input``, and
``csrf_token`` to the context. Apart from that, this backend doesn't create a
Django-flavored environment. It doesn't know about Django context processors,
filters, and tags. In order to use Django-specific APIs, you must configure
them into the environment.

For example, you can create ``myproject/jinja2.py`` with this content::

    from __future__ import absolute_import  # Python 2 only

    from django.contrib.staticfiles.storage import staticfiles_storage
    from django.urls import reverse

    from jinja2 import Environment


    def environment(**options):
        env = Environment(**options)
        env.globals.update({
            'static': staticfiles_storage.url,
            'url': reverse,
        })
        return env

and set the ``'environment'`` option to ``'myproject.jinja2.environment'``.

Then you could use the following constructs in Jinja2 templates:

.. code-block:: html+jinja

    <img src="{{ static('path/to/company-logo.png') }}" alt="Company Logo">

    <a href="{{ url('admin:index') }}">Administration</a>

The concepts of tags and filters exist both in the Django template language
and in Jinja2 but they're used differently. Since Jinja2 supports passing
arguments to callables in templates, many features that require a template tag
or filter in Django templates can be achieved simply by calling a function in
Jinja2 templates, as shown in the example above. Jinja2's global namespace
removes the need for template context processors. The Django template language
doesn't have an equivalent of Jinja2 tests.

Custom backends
---------------

Here's how to implement a custom template backend in order to use another
template system. A template backend is a class that inherits
``django.template.backends.base.BaseEngine``. It must implement
``get_template()`` and optionally ``from_string()``. Here's an example for a
fictional ``foobar`` template library::

    from django.template import TemplateDoesNotExist, TemplateSyntaxError
    from django.template.backends.base import BaseEngine
    from django.template.backends.utils import csrf_input_lazy, csrf_token_lazy

    import foobar


    class FooBar(BaseEngine):

        # Name of the subdirectory containing the templates for this engine
        # inside an installed application.
        app_dirname = 'foobar'

        def __init__(self, params):
            params = params.copy()
            options = params.pop('OPTIONS').copy()
            super(FooBar, self).__init__(params)

            self.engine = foobar.Engine(**options)

        def from_string(self, template_code):
            try:
              return Template(self.engine.from_string(template_code))
            except foobar.TemplateCompilationFailed as exc:
                raise TemplateSyntaxError(exc.args)

        def get_template(self, template_name):
            try:
                return Template(self.engine.get_template(template_name))
            except foobar.TemplateNotFound as exc:
                raise TemplateDoesNotExist(exc.args, backend=self)
            except foobar.TemplateCompilationFailed as exc:
                raise TemplateSyntaxError(exc.args)


    class Template(object):

        def __init__(self, template):
            self.template = template

        def render(self, context=None, request=None):
            if context is None:
                context = {}
            if request is not None:
                context['request'] = request
                context['csrf_input'] = csrf_input_lazy(request)
                context['csrf_token'] = csrf_token_lazy(request)
            return self.template.render(context)

See `DEP 182`_ for more information.

.. _template-debug-integration:

Debug integration for custom engines
------------------------------------

The Django debug page has hooks to provide detailed information when a template
error arises. Custom template engines can use these hooks to enhance the
traceback information that appears to users. The following hooks are available:

.. _template-postmortem:

Template postmortem
~~~~~~~~~~~~~~~~~~~

The postmortem appears when :exc:`~django.template.TemplateDoesNotExist` is
raised. It lists the template engines and loaders that were used when trying
to find a given template. For example, if two Django engines are configured,
the postmortem will appear like:

.. image:: _images/postmortem.png

Custom engines can populate the postmortem by passing the ``backend`` and
``tried`` arguments when raising :exc:`~django.template.TemplateDoesNotExist`.
Backends that use the postmortem :ref:`should specify an origin
<template-origin-api>` on the template object.

Contextual line information
~~~~~~~~~~~~~~~~~~~~~~~~~~~

If an error happens during template parsing or rendering, Django can display
the line the error happened on. For example:

.. image:: _images/template-lines.png

Custom engines can populate this information by setting a ``template_debug``
attribute on exceptions raised during parsing and rendering. This attribute
is a :class:`dict` with the following values:

* ``'name'``: The name of the template in which the exception occurred.

* ``'message'``: The exception message.

* ``'source_lines'``: The lines before, after, and including the line the
  exception occurred on. This is for context, so it shouldn't contain more than
  20 lines or so.

* ``'line'``: The line number on which the exception occurred.

* ``'before'``: The content on the error line before the token that raised the
  error.

* ``'during'``: The token that raised the error.

* ``'after'``: The content on the error line after the token that raised the
  error.

* ``'total'``: The number of lines in ``source_lines``.

* ``'top'``: The line number where ``source_lines`` starts.

* ``'bottom'``: The line number where ``source_lines`` ends.

Given the above template error, ``template_debug`` would look like::

    {
        'name': '/path/to/template.html',
        'message': "Invalid block tag: 'syntax'",
        'source_lines': [
            (1, 'some\n'),
            (2, 'lines\n'),
            (3, 'before\n'),
            (4, 'Hello {% syntax error %} {{ world }}\n'),
            (5, 'some\n'),
            (6, 'lines\n'),
            (7, 'after\n'),
            (8, ''),
        ],
        'line': 4,
        'before': 'Hello ',
        'during': '{% syntax error %}',
        'after': ' {{ world }}\n',
        'total': 9,
        'bottom': 9,
        'top': 1,
    }

.. _template-origin-api:

Origin API and 3rd-party integration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Django templates have an :class:`~django.template.base.Origin` object available
through the ``template.origin`` attribute. This enables debug information to be
displayed in the :ref:`template postmortem <template-postmortem>`, as well as
in 3rd-party libraries, like the `Django Debug Toolbar`_.

Custom engines can provide their own ``template.origin`` information by
creating an object that specifies the following attributes:

* ``'name'``: The full path to the template.

* ``'template_name'``: The relative path to the template as passed into the
  the template loading methods.

* ``'loader_name'``: An optional string identifying the function or class used
  to load the template, e.g. ``django.template.loaders.filesystem.Loader``.

.. currentmodule:: django.template

.. _template-language-intro:

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

.. highlightlang:: html+django

Syntax
------

.. admonition:: About this section

    This is an overview of the Django template language's syntax. For details
    see the :doc:`language syntax reference </ref/templates/language>`.

A Django template is simply a text document or a Python string marked-up using
the Django template language. Some constructs are recognized and interpreted
by the template engine. The main ones are variables and tags.

A template is rendered with a context. Rendering replaces variables with their
values, which are looked up in the context, and executes tags. Everything else
is output as is.

The syntax of the Django template language involves four constructs.

Variables
~~~~~~~~~

A variable outputs a value from the context, which is a dict-like object
mapping keys to values.

Variables are surrounded by ``{{`` and ``}}`` like this::

    My first name is {{ first_name }}. My last name is {{ last_name }}.

With a context of ``{'first_name': 'John', 'last_name': 'Doe'}``, this
template renders to::

    My first name is John. My last name is Doe.

Dictionary lookup, attribute lookup and list-index lookups are implemented
with a dot notation::

    {{ my_dict.key }}
    {{ my_object.attribute }}
    {{ my_list.0 }}

If a variable resolves to a callable, the template system will call it with no
arguments and use its result instead of the callable.

Tags
~~~~

Tags provide arbitrary logic in the rendering process.

This definition is deliberately vague. For example, a tag can output content,
serve as a control structure e.g. an "if" statement or a "for" loop, grab
content from a database, or even enable access to other template tags.

Tags are surrounded by ``{%`` and ``%}`` like this::

    {% csrf_token %}

Most tags accept arguments::

    {% cycle 'odd' 'even' %}

Some tags require beginning and ending tags::

    {% if user.is_authenticated %}Hello, {{ user.username }}.{% endif %}

A :ref:`reference of built-in tags <ref-templates-builtins-tags>` is
available as well as :ref:`instructions for writing custom tags
<howto-writing-custom-template-tags>`.

Filters
~~~~~~~

Filters transform the values of variables and tag arguments.

They look like this::

     {{ django|title }}

With a context of ``{'django': 'the web framework for perfectionists with
deadlines'}``, this template renders to::

    The Web Framework For Perfectionists With Deadlines

Some filters take an argument::

    {{ my_date|date:"Y-m-d" }}

A :ref:`reference of built-in filters <ref-templates-builtins-filters>` is
available as well as :ref:`instructions for writing custom filters
<howto-writing-custom-template-filters>`.

Comments
~~~~~~~~

Comments look like this::

    {# this won't be rendered #}

A :ttag:`{% comment %} <comment>` tag provides multi-line comments.

Components
----------

.. admonition:: About this section

    This is an overview of the Django template language's APIs. For details
    see the :doc:`API reference </ref/templates/api>`.

Engine
~~~~~~

:class:`django.template.Engine` encapsulates an instance of the Django
template system. The main reason for instantiating an
:class:`~django.template.Engine` directly is to use the Django template
language outside of a Django project.

:class:`django.template.backends.django.DjangoTemplates` is a thin wrapper
adapting :class:`django.template.Engine` to Django's template backend API.

Template
~~~~~~~~

:class:`django.template.Template` represents a compiled template.
Templates are obtained with :meth:`Engine.get_template()
<django.template.Engine.get_template>` or :meth:`Engine.from_string()
<django.template.Engine.from_string>`

Likewise ``django.template.backends.django.Template`` is a thin wrapper
adapting :class:`django.template.Template` to the common template API.

Context
~~~~~~~

:class:`django.template.Context` holds some metadata in addition to the
context data. It is passed to :meth:`Template.render()
<django.template.Template.render>` for rendering a template.

:class:`django.template.RequestContext` is a subclass of
:class:`~django.template.Context` that stores the current
:class:`~django.http.HttpRequest` and runs template context processors.

The common API doesn't have an equivalent concept. Context data is passed in a
plain :class:`dict` and the current :class:`~django.http.HttpRequest` is passed
separately if needed.

Loaders
~~~~~~~

Template loaders are responsible for locating templates, loading them, and
returning :class:`~django.template.Template` objects.

Django provides several :ref:`built-in template loaders <template-loaders>`
and supports :ref:`custom template loaders <custom-template-loaders>`.

Context processors
~~~~~~~~~~~~~~~~~~

Context processors are functions that receive the current
:class:`~django.http.HttpRequest` as an argument and return a :class:`dict` of
data to be added to the rendering context.

Their main use is to add common data shared by all templates to the context
without repeating code in every view.

Django provides many :ref:`built-in context processors <context-processors>`.
Implementing a custom context processor is as simple as defining a function.

.. _Jinja2: http://jinja.pocoo.org/
.. _DEP 182: https://github.com/django/deps/blob/master/final/0182-multiple-template-engines.rst
.. _Django Debug Toolbar: https://github.com/django-debug-toolbar/django-debug-toolbar