Django contains a registry of installed applications that stores configuration and provides introspection. It also maintains a list of available models.
>>> from django.apps import apps >>> apps.get_app_config('admin').verbose_name 'Admin'
Projects and applications¶
Django has historically used the term project to describe an installation of Django. A project is defined primarily by a settings module.
The term application describes a Python package that provides some set of features. Applications may be reused in various projects.
This terminology is somewhat confusing these days as it became common to use the phrase “web app” to describe what equates to a Django project.
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_CLASSES 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.
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.
For application users¶
If you’re using “Rock ’n’ roll” in a project called anthology, but you want it to show up as “Gypsy jazz” instead, you can provide your own configuration:
# anthology/apps.py from rock_n_roll.apps import RockNRollConfig class GypsyJazzConfig(RockNRollConfig): verbose_name = "Gypsy jazz" # anthology/settings.py INSTALLED_APPS = [ 'anthology.apps.GypsyJazzConfig', # ... ]
Again, defining project-specific configuration classes in a submodule called apps is a convention, not a requirement.
- 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.
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.
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.
Human-readable name for the application, e.g. “Administration”.
This attribute defaults to label.title().
Filesystem path to the application directory, e.g. '/usr/lib/python2.7/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.
Root module for the application, e.g. <module 'django.contrib.admin' from 'django/contrib/admin/__init__.pyc'>.
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.
Subclasses can override this method to perform initialization tasks such as registering signals. It is called as soon as the registry is fully populated.
You cannot import models in modules that define application configuration classes, but you can use get_model() to access a model class by name, like this:
def ready(self): MyModel = self.get_model('MyModel')
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!
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.
Namespace packages as apps (Python 3.3+)¶
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:
- The namespace package actually has only a single location (i.e. is not spread across more than one directory.)
- The 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.
The application registry provides the following public API. Methods that aren’t listed below are considered private and may change without notice.
Boolean attribute that is set to True when the registry is fully populated.
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.
How applications are loaded¶
When Django starts, django.setup() is responsible for populating the application registry.
Configures Django by:
- Loading the settings.
- Setting up logging.
- Initializing the application registry.
This function is called automatically:
- When running an HTTP server via Django’s WSGI support.
- When invoking a management command.
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.
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'.