Django contains a registry of installed applications that stores configuration and provides introspection. It also maintains a list of available models.
This registry is simply called apps
and it’s available in
django.apps
:
>>> from django.apps import apps
>>> apps.get_app_config('admin').verbose_name
'Admin'
The term project describes a Django web application. The project Python
package is defined primarily by a settings module, but it usually contains
other things. For example, when you run django-admin startproject mysite
you’ll get a mysite
project directory that contains a mysite
Python
package with settings.py
, urls.py
, and wsgi.py
. The project package
is often extended to include things like fixtures, CSS, and templates which
aren’t tied to a particular application.
A project’s root directory (the one that contains manage.py
) is usually
the container for all of a project’s applications which aren’t installed
separately.
The term application describes a Python package that provides some set of features. Applications may be reused in various projects.
Applications include some combination of models, views, templates, template
tags, static files, URLs, middleware, etc. They’re generally wired into
projects with the INSTALLED_APPS
setting and optionally with other
mechanisms such as URLconfs, the MIDDLEWARE
setting, or template
inheritance.
It is important to understand that a Django application is just a set of code
that interacts with various parts of the framework. There’s no such thing as
an Application
object. However, there’s a few places where Django needs to
interact with installed applications, mainly for configuration and also for
introspection. That’s why the application registry maintains metadata in an
AppConfig
instance for each installed application.
There’s no restriction that a project package can’t also be considered an
application and have models, etc. (which would require adding it to
INSTALLED_APPS
).
To configure an application, subclass AppConfig
and put
the dotted path to that subclass in INSTALLED_APPS
.
When INSTALLED_APPS
simply contains the dotted path to an
application module, Django checks for a default_app_config
variable in
that module.
If it’s defined, it’s the dotted path to the AppConfig
subclass for that application.
If there is no default_app_config
, Django uses the base
AppConfig
class.
default_app_config
allows applications that predate Django 1.7 such as
django.contrib.admin
to opt-in to AppConfig
features
without requiring users to update their INSTALLED_APPS
.
New applications should avoid default_app_config
. Instead they should
require the dotted path to the appropriate AppConfig
subclass to be configured explicitly in INSTALLED_APPS
.
If you’re creating a pluggable app called “Rock ’n’ roll”, here’s how you would provide a proper name for the admin:
# rock_n_roll/apps.py
from django.apps import AppConfig
class RockNRollConfig(AppConfig):
name = 'rock_n_roll'
verbose_name = "Rock ’n’ roll"
You can make your application load this AppConfig
subclass by default as follows:
# rock_n_roll/__init__.py
default_app_config = 'rock_n_roll.apps.RockNRollConfig'
That will cause RockNRollConfig
to be used when INSTALLED_APPS
just contains 'rock_n_roll'
. This allows you to make use of
AppConfig
features without requiring your users to
update their INSTALLED_APPS
setting. Besides this use case, it’s
best to avoid using default_app_config
and instead specify the app config
class in INSTALLED_APPS
as described next.
Of course, you can also tell your users to put
'rock_n_roll.apps.RockNRollConfig'
in their INSTALLED_APPS
setting. You can even provide several different
AppConfig
subclasses with different behaviors and allow
your users to choose one via their INSTALLED_APPS
setting.
The recommended convention is to put the configuration class in a submodule of
the application called apps
. However, this isn’t enforced by Django.
You must include the name
attribute for Django
to determine which application this configuration applies to. You can define
any attributes documented in the AppConfig
API
reference.
Note
If your code imports the application registry in an application’s
__init__.py
, the name apps
will clash with the apps
submodule.
The best practice is to move that code to a submodule and import it. A
workaround is to import the registry under a different name:
from django.apps import apps as django_apps
If you’re using “Rock ’n’ roll” in a project called anthology
, but you
want it to show up as “Jazz Manouche” instead, you can provide your own
configuration:
# anthology/apps.py
from rock_n_roll.apps import RockNRollConfig
class JazzManoucheConfig(RockNRollConfig):
verbose_name = "Jazz Manouche"
# anthology/settings.py
INSTALLED_APPS = [
'anthology.apps.JazzManoucheConfig',
# ...
]
Again, defining project-specific configuration classes in a submodule called
apps
is a convention, not a requirement.
AppConfig
[source]¶Application configuration objects store metadata for an application. Some
attributes can be configured in AppConfig
subclasses. Others are set by Django and read-only.
AppConfig.
name
¶Full Python path to the application, e.g. 'django.contrib.admin'
.
This attribute defines which application the configuration applies to. It
must be set in all AppConfig
subclasses.
It must be unique across a Django project.
AppConfig.
label
¶Short name for the application, e.g. 'admin'
This attribute allows relabeling an application when two applications
have conflicting labels. It defaults to the last component of name
.
It should be a valid Python identifier.
It must be unique across a Django project.
AppConfig.
verbose_name
¶Human-readable name for the application, e.g. “Administration”.
This attribute defaults to label.title()
.
AppConfig.
path
¶Filesystem path to the application directory, e.g.
'/usr/lib/python3.4/dist-packages/django/contrib/admin'
.
In most cases, Django can automatically detect and set this, but you can
also provide an explicit override as a class attribute on your
AppConfig
subclass. In a few situations this is
required; for instance if the app package is a namespace package with
multiple paths.
AppConfig.
module
¶Root module for the application, e.g. <module 'django.contrib.admin' from
'django/contrib/admin/__init__.pyc'>
.
AppConfig.
models_module
¶Module containing the models, e.g. <module 'django.contrib.admin.models'
from 'django/contrib/admin/models.pyc'>
.
It may be None
if the application doesn’t contain a models
module.
Note that the database related signals such as
pre_migrate
and
post_migrate
are only emitted for applications that have a models
module.
AppConfig.
get_model
(model_name)[source]¶Returns the Model
with the given
model_name
. Raises LookupError
if no such model exists in this
application. model_name
is case-insensitive.
AppConfig.
ready
()[source]¶Subclasses can override this method to perform initialization tasks such as registering signals. It is called as soon as the registry is fully populated.
Although you can’t import models at the module-level where
AppConfig
classes are defined, you can import them in
ready()
, using either an import
statement or
get_model()
.
If you’re registering model signals
, you
can refer to the sender by its string label instead of using the model
class itself.
Example:
from django.db.models.signals import pre_save
def ready(self):
# importing model classes
from .models import MyModel # or...
MyModel = self.get_model('MyModel')
# registering signals with the model's string label
pre_save.connect(receiver, sender='app_label.MyModel')
Warning
Although you can access model classes as described above, avoid
interacting with the database in your ready()
implementation.
This includes model methods that execute queries
(save()
,
delete()
, manager methods etc.), and
also raw SQL queries via django.db.connection
. Your
ready()
method will run during startup of every management
command. For example, even though the test database configuration is
separate from the production settings, manage.py test
would still
execute some queries against your production database!
Note
In the usual initialization process, the ready
method is only called
once by Django. But in some corner cases, particularly in tests which
are fiddling with installed applications, ready
might be called more
than once. In that case, either write idempotent methods, or put a flag
on your AppConfig
classes to prevent re-running code which should
be executed exactly one time.
Python versions 3.3 and later support Python packages without an
__init__.py
file. These packages are known as “namespace packages” and may
be spread across multiple directories at different locations on sys.path
(see PEP 420).
Django applications require a single base filesystem path where Django (depending on configuration) will search for templates, static assets, etc. Thus, namespace packages may only be Django applications if one of the following is true:
AppConfig
class used to configure the application
has a path
class attribute, which is the
absolute directory path Django will use as the single base path for the
application.If neither of these conditions is met, Django will raise
ImproperlyConfigured
.
apps
¶The application registry provides the following public API. Methods that aren’t listed below are considered private and may change without notice.
apps.
ready
¶Boolean attribute that is set to True
after the registry is fully
populated and all AppConfig.ready()
methods are called.
apps.
get_app_config
(app_label)¶Returns an AppConfig
for the application with the
given app_label
. Raises LookupError
if no such application
exists.
apps.
is_installed
(app_name)¶Checks whether an application with the given name exists in the registry.
app_name
is the full name of the app, e.g. 'django.contrib.admin'
.
apps.
get_model
(app_label, model_name)¶Returns the Model
with the given app_label
and model_name
. As a shortcut, this method also accepts a single
argument in the form app_label.model_name
. model_name
is
case-insensitive.
Raises LookupError
if no such application or model exists. Raises
ValueError
when called with a single argument that doesn’t contain
exactly one dot.
When Django starts, django.setup()
is responsible for populating the
application registry.
setup
(set_prefix=True)[source]¶Configures Django by:
set_prefix
is True, setting the URL resolver script prefix to
FORCE_SCRIPT_NAME
if defined, or /
otherwise.The ability to set the URL resolver script prefix is new.
This function is called automatically:
It must be called explicitly in other cases, for instance in plain Python scripts.
The application registry is initialized in three stages. At each stage, Django
processes all applications in the order of INSTALLED_APPS
.
First Django imports each item in INSTALLED_APPS
.
If it’s an application configuration class, Django imports the root package
of the application, defined by its name
attribute. If
it’s a Python package, Django creates a default application configuration.
At this stage, your code shouldn’t import any models!
In other words, your applications’ root packages and the modules that define your application configuration classes shouldn’t import any models, even indirectly.
Strictly speaking, Django allows importing models once their application
configuration is loaded. However, in order to avoid needless constraints on
the order of INSTALLED_APPS
, it’s strongly recommended not
import any models at this stage.
Once this stage completes, APIs that operate on application configurations
such as get_app_config()
become usable.
Then Django attempts to import the models
submodule of each application,
if there is one.
You must define or import all models in your application’s models.py
or
models/__init__.py
. Otherwise, the application registry may not be fully
populated at this point, which could cause the ORM to malfunction.
Once this stage completes, APIs that operate on models such as
get_model()
become usable.
Finally Django runs the ready()
method of each application
configuration.
Here are some common problems that you may encounter during initialization:
AppRegistryNotReady
: This happens when
importing an application configuration or a models module triggers code that
depends on the app registry.
For example, ugettext()
uses the app
registry to look up translation catalogs in applications. To translate at
import time, you need ugettext_lazy()
instead. (Using ugettext()
would be a bug,
because the translation would happen at import time, rather than at each
request depending on the active language.)
Executing database queries with the ORM at import time in models modules will also trigger this exception. The ORM cannot function properly until all models are available.
Another common culprit is django.contrib.auth.get_user_model()
. Use
the AUTH_USER_MODEL
setting to reference the User model at import
time.
This exception also happens if you forget to call django.setup()
in
a standalone Python script.
ImportError: cannot import name ...
This happens if the import sequence
ends up in a loop.
To eliminate such problems, you should minimize dependencies between your models modules and do as little work as possible at import time. To avoid executing code at import time, you can move it into a function and cache its results. The code will be executed when you first need its results. This concept is known as “lazy evaluation”.
django.contrib.admin
automatically performs autodiscovery of admin
modules in installed applications. To prevent it, change your
INSTALLED_APPS
to contain
'django.contrib.admin.apps.SimpleAdminConfig'
instead of
'django.contrib.admin'
.
Jun 09, 2017