django/docs/ref/templates/api.txt

.. _ref-templates-api:

====================================================
The Django template language: For Python programmers
====================================================

This document explains the Django template system from a technical
perspective -- how it works and how to extend it. If you're just looking for
reference on the language syntax, see :ref:`topics-templates`.

If you're looking to use the Django template system as part of another
application -- i.e., without the rest of the framework -- make sure to read
the `configuration`_ section later in this document.

.. _configuration: `configuring the template system in standalone mode`_

Basics
======

A **template** is a text document, or a normal Python string, that is marked-up
using the Django template language. A template can contain **block tags** or
**variables**.

A **block tag** is a symbol within a template that does something.

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

Block tags are surrounded by ``"{%"`` and ``"%}"``.

Example template with block tags:

.. code-block:: html+django

    {% if is_logged_in %}Thanks for logging in!{% else %}Please log in.{% endif %}

A **variable** is a symbol within a template that outputs a value.

Variable tags are surrounded by ``"{{"`` and ``"}}"``.

Example template with variables:

.. code-block:: html+django

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

A **context** is a "variable name" -> "variable value" mapping that is passed
to a template.

A template **renders** a context by replacing the variable "holes" with values
from the context and executing all block tags.

Using the template system
=========================

Using the template system in Python is a two-step process:

    * First, you compile the raw template code into a ``Template`` object.
    * Then, you call the ``render()`` method of the ``Template`` object with a
      given context.

Compiling a string
------------------

The easiest way to create a ``Template`` object is by instantiating it
directly. The class lives at ``django.template.Template``. The constructor
takes one argument -- the raw template code::

    >>> from django.template import Template
    >>> t = Template("My name is {{ my_name }}.")
    >>> print t
    <django.template.Template instance>

.. admonition:: Behind the scenes

    The system only parses your raw template code once -- when you create the
    ``Template`` object. From then on, it's stored internally as a "node"
    structure for performance.

    Even the parsing itself is quite fast. Most of the parsing happens via a
    single call to a single, short, regular expression.

Rendering a context
-------------------

Once you have a compiled ``Template`` object, you can render a context -- or
multiple contexts -- with it. The ``Context`` class lives at
``django.template.Context``, and the constructor takes two (optional)
arguments:

    * A dictionary mapping variable names to variable values.

    * The name of the current application. This application name is used
      to help :ref:`resolve namespaced URLs<topics-http-reversing-url-namespaces>`.
      If you're not using namespaced URLs, you can ignore this argument.

Call the ``Template`` object's ``render()`` method with the context to "fill" the
template::

    >>> from django.template import Context, Template
    >>> t = Template("My name is {{ my_name }}.")

    >>> c = Context({"my_name": "Adrian"})
    >>> t.render(c)
    "My name is Adrian."

    >>> c = Context({"my_name": "Dolores"})
    >>> t.render(c)
    "My name is Dolores."

Variable names must consist of any letter (A-Z), any digit (0-9), an underscore
or a dot.

Dots have a special meaning in template rendering. A dot in a variable name
signifies **lookup**. Specifically, when the template system encounters a dot
in a variable name, it tries the following lookups, in this order:

    * Dictionary lookup. Example: ``foo["bar"]``
    * Attribute lookup. Example: ``foo.bar``
    * Method call. Example: ``foo.bar()``
    * List-index lookup. Example: ``foo[bar]``

The template system uses the first lookup type that works. It's short-circuit
logic.

Here are a few examples::

    >>> from django.template import Context, Template
    >>> t = Template("My name is {{ person.first_name }}.")
    >>> d = {"person": {"first_name": "Joe", "last_name": "Johnson"}}
    >>> t.render(Context(d))
    "My name is Joe."

    >>> class PersonClass: pass
    >>> p = PersonClass()
    >>> p.first_name = "Ron"
    >>> p.last_name = "Nasty"
    >>> t.render(Context({"person": p}))
    "My name is Ron."

    >>> class PersonClass2:
    ...     def first_name(self):
    ...         return "Samantha"
    >>> p = PersonClass2()
    >>> t.render(Context({"person": p}))
    "My name is Samantha."

    >>> t = Template("The first stooge in the list is {{ stooges.0 }}.")
    >>> c = Context({"stooges": ["Larry", "Curly", "Moe"]})
    >>> t.render(c)
    "The first stooge in the list is Larry."

Method lookups are slightly more complex than the other lookup types. Here are
some things to keep in mind:

    * If, during the method lookup, a method raises an exception, the exception
      will be propagated, unless the exception has an attribute
      ``silent_variable_failure`` whose value is ``True``. If the exception
      *does* have a ``silent_variable_failure`` attribute, the variable will
      render as an empty string. Example::

        >>> t = Template("My name is {{ person.first_name }}.")
        >>> class PersonClass3:
        ...     def first_name(self):
        ...         raise AssertionError, "foo"
        >>> p = PersonClass3()
        >>> t.render(Context({"person": p}))
        Traceback (most recent call last):
        ...
        AssertionError: foo

        >>> class SilentAssertionError(Exception):
        ...     silent_variable_failure = True
        >>> class PersonClass4:
        ...     def first_name(self):
        ...         raise SilentAssertionError
        >>> p = PersonClass4()
        >>> t.render(Context({"person": p}))
        "My name is ."

      Note that ``django.core.exceptions.ObjectDoesNotExist``, which is the
      base class for all Django database API ``DoesNotExist`` exceptions, has
      ``silent_variable_failure = True``. So if you're using Django templates
      with Django model objects, any ``DoesNotExist`` exception will fail
      silently.

    * A method call will only work if the method has no required arguments.
      Otherwise, the system will move to the next lookup type (list-index
      lookup).

    * Obviously, some methods have side effects, and it'd be either foolish or
      a security hole to allow the template system to access them.

      A good example is the ``delete()`` method on each Django model object.
      The template system shouldn't be allowed to do something like this::

        I will now delete this valuable data. {{ data.delete }}

      To prevent this, set a function attribute ``alters_data`` on the method.
      The template system won't execute a method if the method has
      ``alters_data=True`` set. The dynamically-generated ``delete()`` and
      ``save()`` methods on Django model objects get ``alters_data=True``
      automatically. Example::

        def sensitive_function(self):
            self.database_record.delete()
        sensitive_function.alters_data = True

.. _invalid-template-variables:

How invalid variables are handled
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Generally, if a variable doesn't exist, the template system inserts the
value of the :setting:`TEMPLATE_STRING_IF_INVALID` setting, which is set to
``''`` (the empty string) by default.

Filters that are applied to an invalid variable will only be applied if
:setting:`TEMPLATE_STRING_IF_INVALID` is set to ``''`` (the empty string). If
:setting:`TEMPLATE_STRING_IF_INVALID` is set to any other value, variable
filters will be ignored.

This behavior is slightly different for the ``if``, ``for`` and ``regroup``
template tags. If an invalid variable is provided to one of these template
tags, the variable will be interpreted as ``None``. Filters are always
applied to invalid variables within these template tags.

If :setting:`TEMPLATE_STRING_IF_INVALID` contains a ``'%s'``, the format marker will
be replaced with the name of the invalid variable.

.. admonition:: For debug purposes only!

    While :setting:`TEMPLATE_STRING_IF_INVALID` can be a useful debugging tool,
    it is a bad idea to turn it on as a 'development default'.

    Many templates, including those in the Admin site, rely upon the
    silence of the template system when a non-existent variable is
    encountered. If you assign a value other than ``''`` to
    :setting:`TEMPLATE_STRING_IF_INVALID`, you will experience rendering
    problems with these templates and sites.

    Generally, :setting:`TEMPLATE_STRING_IF_INVALID` should only be enabled
    in order to debug a specific template problem, then cleared
    once debugging is complete.

Playing with Context objects
----------------------------

Most of the time, you'll instantiate ``Context`` objects by passing in a
fully-populated dictionary to ``Context()``. But you can add and delete items
from a ``Context`` object once it's been instantiated, too, using standard
dictionary syntax::

    >>> c = Context({"foo": "bar"})
    >>> c['foo']
    'bar'
    >>> del c['foo']
    >>> c['foo']
    ''
    >>> c['newvariable'] = 'hello'
    >>> c['newvariable']
    'hello'

A ``Context`` object is a stack. That is, you can ``push()`` and ``pop()`` it.
If you ``pop()`` too much, it'll raise
``django.template.ContextPopException``::

    >>> c = Context()
    >>> c['foo'] = 'first level'
    >>> c.push()
    >>> c['foo'] = 'second level'
    >>> c['foo']
    'second level'
    >>> c.pop()
    >>> c['foo']
    'first level'
    >>> c['foo'] = 'overwritten'
    >>> c['foo']
    'overwritten'
    >>> c.pop()
    Traceback (most recent call last):
    ...
    django.template.ContextPopException

Using a ``Context`` as a stack comes in handy in some custom template tags, as
you'll see below.

.. _subclassing-context-requestcontext:

Subclassing Context: RequestContext
-----------------------------------

Django comes with a special ``Context`` class,
``django.template.RequestContext``, that acts slightly differently than the
normal ``django.template.Context``. The first difference is that it takes an
:class:`~django.http.HttpRequest` as its first argument. For example::

    c = RequestContext(request, {
        'foo': 'bar',
    })

The second difference is that it automatically populates the context with a few
variables, according to your :setting:`TEMPLATE_CONTEXT_PROCESSORS` setting.

The :setting:`TEMPLATE_CONTEXT_PROCESSORS` setting is a tuple of callables --
called **context processors** -- that take a request object as their argument
and return a dictionary of items to be merged into the context. By default,
:setting:`TEMPLATE_CONTEXT_PROCESSORS` is set to::

    ("django.core.context_processors.auth",
    "django.core.context_processors.debug",
    "django.core.context_processors.i18n",
    "django.core.context_processors.media",
    "django.contrib.messages.context_processors.messages")

.. versionadded:: 1.2
   In addition to these, ``RequestContext`` always uses
   ``'django.core.context_processors.csrf'``.  This is a security
   related context processor required by the admin and other contrib apps, and,
   in case of accidental misconfiguration, it is deliberately hardcoded in and
   cannot be turned off by the :setting:`TEMPLATE_CONTEXT_PROCESSORS` setting.

.. versionadded:: 1.2
   The ``'messages'`` context processor was added.  For more information, see
   the :ref:`messages documentation <ref-contrib-messages>`.

Each processor is applied in order. That means, if one processor adds a
variable to the context and a second processor adds a variable with the same
name, the second will override the first. The default processors are explained
below.

Also, you can give ``RequestContext`` a list of additional processors, using the
optional, third positional argument, ``processors``. In this example, the
``RequestContext`` instance gets a ``ip_address`` variable::

    def ip_address_processor(request):
        return {'ip_address': request.META['REMOTE_ADDR']}

    def some_view(request):
        # ...
        c = RequestContext(request, {
            'foo': 'bar',
        }, [ip_address_processor])
        return HttpResponse(t.render(c))

.. note::
    If you're using Django's ``render_to_response()`` shortcut to populate a
    template with the contents of a dictionary, your template will be passed a
    ``Context`` instance by default (not a ``RequestContext``). To use a
    ``RequestContext`` in your template rendering, pass an optional third
    argument to ``render_to_response()``: a ``RequestContext``
    instance. Your code might look like this::

        def some_view(request):
            # ...
            return render_to_response('my_template.html',
                                      my_data_dictionary,
                                      context_instance=RequestContext(request))

Here's what each of the default processors does:

django.core.context_processors.auth
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If :setting:`TEMPLATE_CONTEXT_PROCESSORS` contains this processor, every
``RequestContext`` will contain these three variables:

    * ``user`` -- An ``auth.User`` instance representing the currently
      logged-in user (or an ``AnonymousUser`` instance, if the client isn't
      logged in).

    * ``messages`` -- A list of messages (as strings) that have been set
      via the :ref:`messages framework <ref-contrib-messages>`.

    * ``perms`` -- An instance of
      ``django.core.context_processors.PermWrapper``, representing the
      permissions that the currently logged-in user has.

.. versionchanged:: 1.2
   Prior to version 1.2, the ``messages`` variable was a lazy accessor for
   ``user.get_and_delete_messages()``. It has been changed to include any
   messages added via the :ref:`messages framework <ref-contrib-messages`.

django.core.context_processors.debug
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If :setting:`TEMPLATE_CONTEXT_PROCESSORS` contains this processor, every
``RequestContext`` will contain these two variables -- but only if your
:setting:`DEBUG` setting is set to ``True`` and the request's IP address
(``request.META['REMOTE_ADDR']``) is in the :setting:`INTERNAL_IPS` setting:

    * ``debug`` -- ``True``. You can use this in templates to test whether
      you're in :setting:`DEBUG` mode.
    * ``sql_queries`` -- A list of ``{'sql': ..., 'time': ...}`` dictionaries,
      representing every SQL query that has happened so far during the request
      and how long it took. The list is in order by query.

django.core.context_processors.i18n
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If :setting:`TEMPLATE_CONTEXT_PROCESSORS` contains this processor, every
``RequestContext`` will contain these two variables:

    * ``LANGUAGES`` -- The value of the :setting:`LANGUAGES` setting.
    * ``LANGUAGE_CODE`` -- ``request.LANGUAGE_CODE``, if it exists. Otherwise,
      the value of the :setting:`LANGUAGE_CODE` setting.

See :ref:`topics-i18n` for more.

django.core.context_processors.media
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 1.0

If :setting:`TEMPLATE_CONTEXT_PROCESSORS` contains this processor, every
``RequestContext`` will contain a variable ``MEDIA_URL``, providing the
value of the :setting:`MEDIA_URL` setting.

django.core.context_processors.csrf
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 1.2

This processor adds a token that is needed by the ``csrf_token`` template tag
for protection against :ref:`Cross Site Request Forgeries <ref-contrib-csrf>`.

django.core.context_processors.request
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If :setting:`TEMPLATE_CONTEXT_PROCESSORS` contains this processor, every
``RequestContext`` will contain a variable ``request``, which is the current
:class:`~django.http.HttpRequest`. Note that this processor is not enabled by default;
you'll have to activate it.

django.contrib.messages.context_processors.messages
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If :setting:`TEMPLATE_CONTEXT_PROCESSORS` contains this processor, every
``RequestContext`` will contain a single additional variable:

    * ``messages`` -- A list of messages (as strings) that have been set
      via the user model (using ``user.message_set.create``) or through
      the :ref:`messages framework <ref-contrib-messages>`.

.. versionadded:: 1.2
   This template context variable was previously supplied by the ``'auth'``
   context processor.  For backwards compatibility the ``'auth'`` context
   processor will continue to supply the ``messages`` variable until Django
   1.4.  If you use the ``messages`` variable, your project will work with
   either (or both) context processors, but it is recommended to add
   ``django.contrib.messages.context_processors.messages`` so your project
   will be prepared for the future upgrade.

Writing your own context processors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A context processor has a very simple interface: It's just a Python function
that takes one argument, an ``HttpRequest`` object, and returns a dictionary
that gets added to the template context. Each context processor *must* return
a dictionary.

Custom context processors can live anywhere in your code base. All Django cares
about is that your custom context processors are pointed-to by your
:setting:`TEMPLATE_CONTEXT_PROCESSORS` setting.

Loading templates
-----------------

Generally, you'll store templates in files on your filesystem rather than using
the low-level ``Template`` API yourself. Save templates in a directory
specified as a **template directory**.

Django searches for template directories in a number of places, depending on
your template-loader settings (see "Loader types" below), but the most basic
way of specifying template directories is by using the :setting:`TEMPLATE_DIRS`
setting.

The TEMPLATE_DIRS setting
~~~~~~~~~~~~~~~~~~~~~~~~~

Tell Django what your template directories are by using the
:setting:`TEMPLATE_DIRS` setting in your settings file. This should be set to a
list or tuple of strings that contain full paths to your template
directory(ies). Example::

    TEMPLATE_DIRS = (
        "/home/html/templates/lawrence.com",
        "/home/html/templates/default",
    )

Your templates can go anywhere you want, as long as the directories and
templates are readable by the Web server. They can have any extension you want,
such as ``.html`` or ``.txt``, or they can have no extension at all.

Note that these paths should use Unix-style forward slashes, even on Windows.

.. _ref-templates-api-the-python-api:

The Python API
~~~~~~~~~~~~~~

Django has two ways to load templates from files:

.. function:: django.template.loader.get_template(template_name)

    ``get_template`` returns the compiled template (a ``Template`` object) for
    the template with the given name. If the template doesn't exist, it raises
    ``django.template.TemplateDoesNotExist``.

.. function:: django.template.loader.select_template(template_name_list)

    ``select_template`` is just like ``get_template``, except it takes a list
    of template names. Of the list, it returns the first template that exists.

For example, if you call ``get_template('story_detail.html')`` and have the
above :setting:`TEMPLATE_DIRS` setting, here are the files Django will look for,
in order:

    * ``/home/html/templates/lawrence.com/story_detail.html``
    * ``/home/html/templates/default/story_detail.html``

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

    * ``/home/html/templates/lawrence.com/story_253_detail.html``
    * ``/home/html/templates/default/story_253_detail.html``
    * ``/home/html/templates/lawrence.com/story_detail.html``
    * ``/home/html/templates/default/story_detail.html``

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

.. admonition:: Tip

    You can use ``select_template()`` for super-flexible "templatability." 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.

Using subdirectories
~~~~~~~~~~~~~~~~~~~~

It's possible -- and preferable -- to organize templates in subdirectories of
the template directory. 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:`TEMPLATE_DIRS` setting from above, this example
``get_template()`` call will attempt to load the following templates:

    * ``/home/html/templates/lawrence.com/news/story_detail.html``
    * ``/home/html/templates/default/news/story_detail.html``

.. _template-loaders:

Loader types
~~~~~~~~~~~~

By default, Django uses a filesystem-based template loader, but Django comes
with a few other template loaders, which know how to load templates from other
sources.

Some of these other loaders are disabled by default, but you can activate them
by editing your :setting:`TEMPLATE_LOADERS` setting. :setting:`TEMPLATE_LOADERS`
should be a tuple of strings, where each string represents a template loader.
Here are the template loaders that come with Django:

``django.template.loaders.filesystem.Loader``
    Loads templates from the filesystem, according to :setting:`TEMPLATE_DIRS`.
    This loader is enabled by default.

``django.template.loaders.app_directories.Loader``
    Loads templates from Django apps on the filesystem. For each app in
    :setting:`INSTALLED_APPS`, the loader looks for a ``templates``
    subdirectory. If the directory exists, Django looks for templates in there.

    This means you can store templates with your individual apps. This also
    makes it easy to distribute Django apps with default templates.

    For example, for this setting::

        INSTALLED_APPS = ('myproject.polls', 'myproject.music')

    ...then ``get_template('foo.html')`` will look for templates in these
    directories, in this order:

        * ``/path/to/myproject/polls/templates/foo.html``
        * ``/path/to/myproject/music/templates/foo.html``

    Note that the loader performs an optimization when it is first imported: It
    caches a list of which :setting:`INSTALLED_APPS` packages have a
    ``templates`` subdirectory.

    This loader is enabled by default.

``django.template.loaders.eggs.Loader``
    Just like ``app_directories`` above, but it loads templates from Python
    eggs rather than from the filesystem.

    This loader is disabled by default.

``django.template.loaders.cached.Loader``
    By default, the templating system will read and compile your templates every
    time they need to be rendered. While the Django templating system is quite
    fast, the overhead from reading and compiling templates can add up.

    The cached template loader is a class-based loader that you configure with
    a list of other loaders that it should wrap. The wrapped loaders are used to
    locate unknown templates when they are first encountered. The cached loader
    then stores the compiled ``Template`` in memory. The cached ``Template``
    instance is returned for subsequent requests to load the same template.

    For example, to enable template caching with the ``filesystem`` and
    ``app_directories`` template loaders you might use the following settings::

        TEMPLATE_LOADERS = (
            ('django.template.loaders.cached.Loader', (
                'django.template.loaders.filesystem.Loader',
                'django.template.loaders.app_directories.Loader',
            )),
        )

    .. note::
        All of the built-in Django template tags are safe to use with the cached
        loader, but if you're using custom template tags that come from third
        party packages, or that you wrote yourself, you should ensure that the
        ``Node`` implementation for each tag is thread-safe. For more
        information, see
        :ref:`template tag thread safety considerations<template_tag_thread_safety>`.

    This loader is disabled by default.

Django uses the template loaders in order according to the
:setting:`TEMPLATE_LOADERS` setting. It uses each loader until a loader finds a
match.

The ``render_to_string()`` shortcut
===================================

To cut down on the repetitive nature of loading and rendering
templates, Django provides a shortcut function which largely
automates the process: ``render_to_string()`` in
``django.template.loader``, which loads a template, renders it and
returns the resulting string::

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

The ``render_to_string`` shortcut takes one required argument --
``template_name``, which should be the name of the template to load
and render -- and two optional arguments:

    dictionary
        A dictionary to be used as variables and values for the
        template's context. This can also be passed as the second
        positional argument.

    context_instance
        An instance of ``Context`` or a subclass (e.g., an instance of
        ``RequestContext``) to use as the template's context. This can
        also be passed as the third positional argument.

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

Configuring the template system in standalone mode
==================================================

.. note::

    This section is only of interest to people trying to use the template
    system as an output component in another application. If you're using the
    template system as part of a Django application, nothing here applies to
    you.

Normally, Django will load all the configuration information it needs from its
own default configuration file, combined with the settings in the module given
in the :setting:`DJANGO_SETTINGS_MODULE` environment variable. But if you're
using the template system independently of the rest of Django, the environment
variable approach isn't very convenient, because you probably want to configure
the template system in line with the rest of your application rather than
dealing with settings files and pointing to them via environment variables.

To solve this problem, you need to use the manual configuration option described
in :ref:`settings-without-django-settings-module`. Simply import the appropriate
pieces of the templating system and then, *before* you call any of the
templating functions, call ``django.conf.settings.configure()`` with any
settings you wish to specify. You might want to consider setting at least
:setting:`TEMPLATE_DIRS` (if you're going to use template loaders),
:setting:`DEFAULT_CHARSET` (although the default of ``utf-8`` is probably fine)
and :setting:`TEMPLATE_DEBUG`. All available settings are described in the
:ref:`settings documentation <ref-settings>`, and any setting starting with
``TEMPLATE_`` is of obvious interest.

.. _topic-template-alternate-language:

Using an alternative template language
======================================

.. versionadded 1.2

The Django ``Template`` and ``Loader`` classes implement a simple API for
loading and rendering templates. By providing some simple wrapper classes that
implement this API we can use third party template systems like `Jinja2
<http://jinja.pocoo.org/2/>`_ or `Cheetah <http://www.cheetahtemplate.org/>`_. This
allows us to use third-party template libraries without giving up useful Django
features like the Django ``Context`` object and handy shortcuts like
``render_to_response()``.

The core component of the Django templating system is the ``Template`` class.
This class has a very simple interface: it has a constructor that takes a single
positional argument specifying the template string, and a ``render()`` method
that takes a ``django.template.context.Context`` object and returns a string
containing the rendered response.

Suppose we're using a template language that defines a ``Template`` object with
a ``render()`` method that takes a dictionary rather than a ``Context`` object.
We can write a simple wrapper that implements the Django ``Template`` interface::

    import some_template_language
    class Template(some_template_language.Template):
        def render(self, context):
            # flatten the Django Context into a single dictionary.
            context_dict = {}
            for d in context.dicts:
                context_dict.update(d)
            return super(Template, self).render(context_dict)

That's all that's required to make our fictional ``Template`` class compatible
with the Django loading and rendering system!

The next step is to write a ``Loader`` class that returns instances of our custom
template class instead of the default ``django.template.Template``. Custom ``Loader``
classes should inherit from ``django.template.loader.BaseLoader`` and override
the ``load_template_source()`` method, which takes a ``template_name`` argument,
loads the template from disk (or elsewhere), and returns a tuple:
``(template_string, template_origin)``.

The ``load_template()`` method of the ``Loader`` class retrieves the template
string by calling ``load_template_source()``, instantiates a ``Template`` from
the template source, and returns a tuple: ``(template, template_origin)``. Since
this is the method that actually instantiates the ``Template``, we'll need to
override it to use our custom template class instead. We can inherit from the
builtin ``django.template.loaders.app_directories.Loader`` to take advantage of
the ``load_template_source()`` method implemented there::

    from django.template.loaders import app_directories
    class Loader(app_directories.Loader):
        is_usable = True

        def load_template(self, template_name, template_dirs=None):
            source, origin = self.load_template_source(template_name, template_dirs)
            template = Template(source)
            return template, origin

Finally, we need to modify our project settings, telling Django to use our custom
loader. Now we can write all of our templates in our alternative template
language while continuing to use the rest of the Django templating system.