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 the value of the engine’s string_if_invalid configuration option (an empty string, by default). 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 variable can only be called if it has no required arguments. Otherwise, the system will return the value of the engine’s string_if_invalid option.

• 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 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 uncalled 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 engine’s string_if_invalid configuration option, which is set to '' (the empty string) by default.

Filters that are applied to an invalid variable will only be applied if string_if_invalid is set to '' (the empty string). If 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 string_if_invalid contains a '%s', the format marker will be replaced with the name of the invalid variable.

Built-in variables¶

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

Limitations with string literals¶

Django’s template language has no way to escape the characters used for its own syntax. For example, the templatetag tag is required if you need to output character sequences like {% and %}.

A similar issue exists if you want to include these sequences in template filter or tag arguments. For example, when parsing a block tag, Django’s template parser looks for the first occurrence of %} after a {%. This prevents the use of "%}" as a string literal. For example, a TemplateSyntaxError will be raised for the following expressions:

{% include "template.html" tvar="Some string literal with %} in it." %}

{% with tvar="Some string literal with %} in it." %}{% endwith %}

The same issue can be triggered by using a reserved sequence in filter arguments:

{{ some.variable|default:"}}" }}

If you need to use strings with these sequences, store them in template variables or use a custom template tag or filter to workaround the limitation.

Using RequestContext¶

class RequestContext(request, dict_=None, processors=None, use_l10n=None, use_tz=None, autoescape=True)[código-fonte]¶

Django comes with a special Context class, django.template.RequestContext, that acts slightly differently from 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 the engine’s context_processors configuration option.

The context_processors option is a list 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. In the default generated settings file, the default template engine contains the following context processors:

[
    "django.template.context_processors.debug",
    "django.template.context_processors.request",
    "django.contrib.auth.context_processors.auth",
    "django.contrib.messages.context_processors.messages",
]

In addition to these, RequestContext always enables 'django.template.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 in the context_processors option.

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.

from django.template import RequestContext

request_context = RequestContext(request)
request_context.push({"my_name": "Adrian"})

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

from django.http import HttpResponse
from django.template import RequestContext, Template


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


def client_ip_view(request):
    template = Template("{{ title }}: {{ ip_address }}")
    context = RequestContext(
        request,
        {
            "title": "Your IP Address",
        },
        [ip_address_processor],
    )
    return HttpResponse(template.render(context))

Built-in template context processors¶

Here’s what each of the built-in processors does:

Writing your own context processors¶

A context processor has a simple interface: It’s a Python function that takes one argument, an HttpRequest object, and returns a dictionary that gets added to the template context.

For example, to add the DEFAULT_FROM_EMAIL setting to every context:

from django.conf import settings


def from_email(request):
    return {
        "DEFAULT_FROM_EMAIL": settings.DEFAULT_FROM_EMAIL,
    }

Custom context processors can live anywhere in your code base. All Django cares about is that your custom context processors are pointed to by the 'context_processors' option in your TEMPLATES setting — or the context_processors argument of Engine if you’re using it directly.

The DIRS option¶

Tell Django what your template directories are by using the DIRS option in the TEMPLATES setting in your settings file — or the dirs argument of Engine. This should be set to a list of strings that contain full paths to your template directories:

TEMPLATES = [
    {
        "BACKEND": "django.template.backends.django.DjangoTemplates",
        "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.

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 adding a 'loaders' option to your DjangoTemplates backend in the TEMPLATES setting or passing a loaders argument to Engine. loaders should be a list of strings or tuples, where each 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 DIRS.

This loader is enabled by default. However it won’t find any templates until you set DIRS to a non-empty list:

TEMPLATES = [
    {
        "BACKEND": "django.template.backends.django.DjangoTemplates",
        "DIRS": [BASE_DIR / "templates"],
    }
]

You can also override 'DIRS' and specify specific directories for a particular filesystem loader:

TEMPLATES = [
    {
        "BACKEND": "django.template.backends.django.DjangoTemplates",
        "OPTIONS": {
            "loaders": [
                (
                    "django.template.loaders.filesystem.Loader",
                    [BASE_DIR / "templates"],
                ),
            ],
        },
    }
]

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 helps 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 first runs: it caches a list of which INSTALLED_APPS packages have a templates subdirectory.

You can enable this loader by setting APP_DIRS to True:

TEMPLATES = [
    {
        "BACKEND": "django.template.backends.django.DjangoTemplates",
        "APP_DIRS": True,
    }
]

django.template.loaders.cached.Loader

class cached.Loader¶

While the Django template system is quite fast, if it needs to read and compile your templates every time they’re rendered, the overhead from that can add up.

You configure the cached template loader with a list of other loaders that it should wrap. The wrapped loaders are used to locate unknown templates when they’re 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.

This loader is automatically enabled if OPTIONS['loaders'] isn’t specified.

You can manually specify template caching with some custom template loaders using settings like this:

TEMPLATES = [
    {
        "BACKEND": "django.template.backends.django.DjangoTemplates",
        "DIRS": [BASE_DIR / "templates"],
        "OPTIONS": {
            "loaders": [
                (
                    "django.template.loaders.cached.Loader",
                    [
                        "django.template.loaders.filesystem.Loader",
                        "django.template.loaders.app_directories.Loader",
                        "path.to.custom.Loader",
                    ],
                ),
            ],
        },
    }
]

Nota

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.

django.template.loaders.locmem.Loader

class locmem.Loader¶

Carrega templates de um dicionário Python. Isto é útil para realização de testes.

This loader takes a dictionary of templates as its first argument:

TEMPLATES = [
    {
        "BACKEND": "django.template.backends.django.DjangoTemplates",
        "OPTIONS": {
            "loaders": [
                (
                    "django.template.loaders.locmem.Loader",
                    {
                        "index.html": "content here",
                    },
                ),
            ],
        },
    }
]

This loader is disabled by default.

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

Loader methods¶

class Loader[código-fonte]¶

Loads templates from a given source, such as the filesystem or a database.

get_template_sources(template_name)[código-fonte]¶

A method that takes a template_name and yields Origin instances for each possible source.

For example, the filesystem loader may receive 'index.html' as a template_name argument. This method would yield origins for the full path of index.html as it appears in each template directory the loader looks at.

The method doesn’t need to verify that the template exists at a given path, but it should ensure the path is valid. For instance, the filesystem loader makes sure the path lies under a valid template directory.

get_contents(origin)¶

Returns the contents for a template given a Origin instance.

This is where a filesystem loader would read contents from the filesystem, or a database loader would read from the database. If a matching template doesn’t exist, this should raise a TemplateDoesNotExist error.

get_template(template_name, skip=None)[código-fonte]¶

Returns a Template object for a given template_name by looping through results from get_template_sources() and calling get_contents(). This returns the first matching template. If no template is found, TemplateDoesNotExist is raised.

The optional skip argument is a list of origins to ignore when extending templates. This allow templates to extend other templates of the same name. It also used to avoid recursion errors.

In general, it is enough to define get_template_sources() and get_contents() for custom template loaders. get_template() will usually not need to be overridden.