Django contains a registry of installed applications that stores configuration and provides introspection. It also maintains a list of available models.
This registry is called apps and it’s available in django.apps:
>>> from django.apps import apps
>>> apps.get_app_config('admin').verbose_name
'Administration'
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, asgi.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 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, create an apps.py module inside the application, then define a subclass of AppConfig there.
When INSTALLED_APPS contains the dotted path to an application module, by default, if Django finds exactly one AppConfig subclass in the apps.py submodule, it uses that configuration for the application. This behavior may be disabled by setting AppConfig.default to False.
If the apps.py module contains more than one AppConfig subclass, Django will look for a single one where AppConfig.default is True.
If no AppConfig subclass is found, the base AppConfig class will be used.
Alternatively, INSTALLED_APPS may contain the dotted path to a configuration class to specify it explicitly:
INSTALLED_APPS = [
...
'polls.apps.PollsAppConfig',
...
]
# rock_n_roll/apps.py
from django.apps import AppConfig
class RockNRollConfig(AppConfig):
name = 'rock_n_roll'
verbose_name = "Rock ’n’ roll"
RockNRollConfig will be loaded automatically when INSTALLED_APPS contains 'rock_n_roll'. If you need to prevent this, set default to False in the class definition.
You can provide several AppConfig subclasses with different behaviors. To tell Django which one to use by default, set default to True in its definition. If your users want to pick a non-default configuration, they must replace 'rock_n_roll' with the dotted path to that specific class in their INSTALLED_APPS setting.
The AppConfig.name attribute tells Django which application this configuration applies to. You can define any other attribute documented in the AppConfig API reference.
AppConfig subclasses may be defined anywhere. The apps.py convention merely allows Django to load them automatically when INSTALLED_APPS contains the path to an application module rather than the path to a configuration class.
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
In previous versions, a default_app_config variable in the application module was used to identify the default application configuration class.
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',
# ...
]
This example shows project-specific configuration classes located in a submodule called apps.py. This is a convention, not a requirement. AppConfig subclasses may be defined anywhere.
In this situation, INSTALLED_APPS must contain the dotted path to the configuration class because it lives outside of an application and thus cannot be automatically detected.
class AppConfig 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/pythonX.Y/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.default Set this attribute to False to prevent Django from selecting a configuration class automatically. This is useful when apps.py defines only one AppConfig subclass but you don’t want Django to use it by default.
Set this attribute to True to tell Django to select a configuration class automatically. This is useful when apps.py defines more than one AppConfig subclass and you want Django to use one of them by default.
By default, this attribute isn’t set.
AppConfig.default_auto_field The implicit primary key type to add to models within this app. You can use this to keep AutoField as the primary key type for third party applications.
By default, this is the value of DEFAULT_AUTO_FIELD.
AppConfig.module Root module for the application, e.g. <module 'django.contrib.admin' from
'django/contrib/admin/__init__.py'>.
AppConfig.models_module Module containing the models, e.g. <module 'django.contrib.admin.models'
from 'django/contrib/admin/models.py'>.
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_models() Returns an iterable of Model classes for this application.
Requires the app registry to be fully populated.
AppConfig.get_model(model_name, require_ready=True) Returns the Model with the given model_name. model_name is case-insensitive.
Raises LookupError if no such model exists in this application.
Requires the app registry to be fully populated unless the require_ready argument is set to False. require_ready behaves exactly as in apps.get_model().
AppConfig.ready() 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.apps import AppConfig
from django.db.models.signals import pre_save
class RockNRollConfig(AppConfig):
# ...
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 packages without an __init__.py file 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_configs() Returns an iterable of AppConfig instances.
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, require_ready=True) 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.
Requires the app registry to be fully populated unless the require_ready argument is set to False.
Setting require_ready to False allows looking up models while the app registry is being populated, specifically during the second phase where it imports models. Then get_model() has the same effect as importing the model. The main use case is to configure model classes with settings, such as AUTH_USER_MODEL.
When require_ready is False, get_model() returns a model class that may not be fully functional (reverse accessors may be missing, for example) until the app registry is fully populated. For this reason, it’s best to leave require_ready to the default value of True whenever possible.
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.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 looks for an application configuration in an apps.py submodule, or else 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.
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, gettext() uses the app registry to look up translation catalogs in applications. To translate at import time, you need gettext_lazy() instead. (Using gettext() 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.
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'.
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Licensed under the BSD License.
https://docs.djangoproject.com/en/3.2/ref/applications/