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 The Django template language.

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.

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:

{% 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:

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

class django.template.Template

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>

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

render(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 resolve namespaced URLs. 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."

Variables and lookups

Variable names must consist of any letter (A-Z), any digit (0-9), an underscore (but they must not start with an underscore) or a dot.

Dots have a special meaning in template rendering. A dot in a variable name signifies a 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
  • List-index lookup. Example: foo[bar]

Note that “bar” in a template expression like {{ foo.bar }} will be interpreted as a literal string and not using the value of the variable “bar”, if one exists in the template context.

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."

>>> 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."

If any part of the variable is callable, the template system will try calling it. Example:

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

Callable variables are slightly more complex than variables which only require straight lookups. Here are some things to keep in mind:

  • If the variable raises an exception when called, 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 whose value is True, 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 ."
Traceback (most recent call last):
...
AssertionError: foo

    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 variable can only be called if it has no required arguments. Otherwise, the system will return an empty string.

  • Obviously, there can be side effects when calling some variables, 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 an alters_data attribute on the callable variable. The template system won’t call a variable if it has alters_data=True set, and will instead replace the variable with TEMPLATE_STRING_IF_INVALID, unconditionally. 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
  • Occasionally you may want to turn off this feature for other reasons, and tell the template system to leave a variable un-called no matter what. To do so, set a do_not_call_in_templates attribute on the callable with the value True. The template system then will act as if your variable is not callable (allowing you to access attributes of the callable, for example).

How invalid variables are handled

Generally, if a variable doesn’t exist, the template system inserts the value of the 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 TEMPLATE_STRING_IF_INVALID is set to '' (the empty string). If 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 TEMPLATE_STRING_IF_INVALID contains a '%s', the format marker will be replaced with the name of the invalid variable.

For debug purposes only!

While 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 TEMPLATE_STRING_IF_INVALID, you will experience rendering problems with these templates and sites.

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

Builtin variables

Every context contains True, False and None. As you would expect, these variables resolve to the corresponding Python objects.

Before Django 1.5, these variables weren’t a special case, and they resolved to None unless you defined them in the context.

Playing with Context objects

class django.template.Context

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'
pop()
push()
exception django.template.ContextPopException

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
update(other_dict)

In addition to push() and pop(), the Context object also defines an update() method. This works like push() but takes a dictionary as an argument and pushes that dictionary onto the stack instead of an empty one.

>>> c = Context()
>>> c['foo'] = 'first level'
>>> c.update({'foo': 'updated'})
{'foo': 'updated'}
>>> c['foo']
'updated'
>>> c.pop()
{'foo': 'updated'}
>>> c['foo']
'first level'

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

Subclassing Context: RequestContext

class django.template.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 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 TEMPLATE_CONTEXT_PROCESSORS setting.

The 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, TEMPLATE_CONTEXT_PROCESSORS is set to:

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

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 TEMPLATE_CONTEXT_PROCESSORS setting.

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.

When context processors are applied

When you use RequestContext, the variables you supply directly are added first, followed any variables supplied by context processors. This means that a context processor may overwrite a variable you’ve supplied, so take care to avoid variable names which overlap with those supplied by your context processors.

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))

Alternatively, use the render() shortcut which is the same as a call to render_to_response() with a context_instance argument that forces the use of a RequestContext.

Here’s what each of the default processors does:

django.contrib.auth.context_processors.auth

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

  • user – An auth.User instance representing the currently logged-in user (or an AnonymousUser instance, if the client isn’t logged in).
  • perms – An instance of django.contrib.auth.context_processors.PermWrapper, representing the permissions that the currently logged-in user has.

django.core.context_processors.debug

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

  • debug – True. You can use this in templates to test whether you’re in 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 TEMPLATE_CONTEXT_PROCESSORS contains this processor, every RequestContext will contain these two variables:

  • LANGUAGES – The value of the LANGUAGES setting.
  • LANGUAGE_CODE – request.LANGUAGE_CODE, if it exists. Otherwise, the value of the LANGUAGE_CODE setting.

See Internationalization and localization for more.

django.core.context_processors.media

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

django.core.context_processors.static

django.core.context_processors.static()

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

django.core.context_processors.csrf

This processor adds a token that is needed by the csrf_token template tag for protection against Cross Site Request Forgeries.

django.core.context_processors.request

If TEMPLATE_CONTEXT_PROCESSORS contains this processor, every RequestContext will contain a variable request, which is the current HttpRequest. Note that this processor is not enabled by default; you’ll have to activate it.

django.contrib.messages.context_processors.messages

If 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 messages framework.

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 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 TEMPLATE_DIRS setting.

The TEMPLATE_DIRS setting

Tell Django what your template directories are by using the 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.

The Python API

django.template.loader has two functions to load templates from files:

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.

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 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.

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 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

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 TEMPLATE_LOADERS setting. TEMPLATE_LOADERS should be a tuple of strings, where each string represents a template loader class. Here are the template loaders that come with Django:

django.template.loaders.filesystem.Loader

class filesystem.Loader

Loads templates from the filesystem, according to TEMPLATE_DIRS. This loader is enabled by default.

django.template.loaders.app_directories.Loader

class app_directories.Loader

Loads templates from Django apps on the filesystem. For each app in 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 foo.html in these directories, in this order:

  • /path/to/myproject/polls/templates/
  • /path/to/myproject/music/templates/

... and will use the one it finds first.

The order of INSTALLED_APPS is significant! For example, if you want to customize the Django admin, you might choose to override the standard admin/base_site.html template, from django.contrib.admin, with your own admin/base_site.html in myproject.polls. You must then make sure that your myproject.polls comes before django.contrib.admin in INSTALLED_APPS, otherwise django.contrib.admin‘s will be loaded first and yours will be ignored.

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

This loader is enabled by default.

django.template.loaders.eggs.Loader

class 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

class 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 template tag thread safety considerations.

This loader is disabled by default.

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

The render_to_string shortcut

django.template.loader.render_to_string(template_name, dictionary=None, context_instance=None)

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 (or a list of template names, in which case Django will use the first template in the list that exists) – 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 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 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 Using settings without setting 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 TEMPLATE_DIRS (if you’re going to use template loaders), DEFAULT_CHARSET (although the default of utf-8 is probably fine) and TEMPLATE_DEBUG. If you plan to use the url template tag, you will also need to set the ROOT_URLCONF setting. All available settings are described in the settings documentation, and any setting starting with TEMPLATE_ is of obvious interest.

Using an alternative template language

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 or Cheetah. 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 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 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.