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¶
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 theTemplate
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¶
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 one (optional)
argument: a dictionary mapping variable names to variable values. 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 isTrue
. If the exception does have asilent_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 APIDoesNotExist
exceptions, hassilent_variable_failure = True
. So if you’re using Django templates with Django model objects, anyDoesNotExist
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 hasalters_data=True
set. The dynamically-generateddelete()
andsave()
methods on Django model objects getalters_data=True
automatically. Example:def sensitive_function(self): self.database_record.delete() sensitive_function.alters_data = True
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.
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¶
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.core.context_processors.auth",
"django.core.context_processors.debug",
"django.core.context_processors.i18n",
"django.core.context_processors.media")
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 TEMPLATE_CONTEXT_PROCESSORS
contains this processor, every
RequestContext
will contain these three variables:
user
– Anauth.User
instance representing the currently logged-in user (or anAnonymousUser
instance, if the client isn’t logged in).messages
– A list of messages (as strings) for the currently logged-in user. Behind the scenes, this callsrequest.user.get_and_delete_messages()
for every request. That method collects the user’s messages and deletes them from the database.Note that messages are set with
user.message_set.create
.perms
– An instance ofdjango.core.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 inDEBUG
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 theLANGUAGES
setting.LANGUAGE_CODE
–request.LANGUAGE_CODE
, if it exists. Otherwise, the value of theLANGUAGE_CODE
setting.
See Internacionalização 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.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.
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 has two ways to load templates from files:
django.template.loader.get_template(template_name)
get_template
returns the compiled template (aTemplate
object) for the template with the given name. If the template doesn’t exist, it raisesdjango.template.TemplateDoesNotExist
.django.template.loader.select_template(template_name_list)
select_template
is just likeget_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.
Here are the template loaders that come with Django:
django.template.loaders.filesystem.load_template_source
- Loads templates from the filesystem, according to
TEMPLATE_DIRS
. This loader is enabled by default. django.template.loaders.app_directories.load_template_source
Loads templates from Django apps on the filesystem. For each app in
INSTALLED_APPS
, the loader looks for atemplates
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
INSTALLED_APPS
packages have atemplates
subdirectory.This loader is enabled by default.
django.template.loaders.eggs.load_template_source
Just like
app_directories
above, but it loads templates from Python eggs rather than from the filesystem.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¶
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 ofRequestContext
) 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 Usando o settings sem a configuração 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
. All available settings are described in the
settings documentation, and any setting starting with
TEMPLATE_
is of obvious interest.