django/docs/ref/applications.txt

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============
Applications
============
.. module:: django.apps
.. versionadded:: 1.7
Django contains a registry of installed applications that stores configuration
and provides introspection. It also maintains a list of available :doc:`models
</topics/db/models>`.
This registry is simply called :attr:`~django.apps.apps` and it's available in
:mod:`django.apps`::
>>> 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.
.. note::
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 :setting:`INSTALLED_APPS` setting and optionally with other
mechanisms such as URLconfs, the :setting:`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
:class:`~django.apps.AppConfig` instance for each installed application.
Configuring applications
========================
To configure an application, subclass :class:`~django.apps.AppConfig` and put
the dotted path to that subclass in :setting:`INSTALLED_APPS`.
When :setting:`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 :class:`~django.apps.AppConfig`
subclass for that application.
If there is no ``default_app_config``, Django uses the base
:class:`~django.apps.AppConfig` class.
For application authors
-----------------------
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 :class:`~django.apps.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 :setting:`INSTALLED_APPS`
just contains ``'rock_n_roll'``. This allows you to make use of
:class:`~django.apps.AppConfig` features without requiring your users to
update their :setting:`INSTALLED_APPS` setting.
Of course, you can also tell your users to put
``'rock_n_roll.apps.RockNRollConfig'`` in their :setting:`INSTALLED_APPS`
setting. You can even provide several different
:class:`~django.apps.AppConfig` subclasses with different behaviors and allow
your users to choose one via their :setting:`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 :attr:`~django.apps.AppConfig.name` attribute for Django
to determine which application this configuration applies to. You can define
any attributes documented in the :class:`~django.apps.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
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.
Application configuration
=========================
.. class:: AppConfig
Application configuration objects store metadata for an application. Some
attributes can be configured in :class:`~django.apps.AppConfig`
subclasses. Others are set by Django and read-only.
Configurable attributes
-----------------------
.. attribute:: 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 :class:`~django.apps.AppConfig` subclasses.
It must be unique across a Django project.
.. attribute:: AppConfig.label
Short name for the application, e.g. ``'admin'``
This attribute allows relabelling 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.
.. attribute:: AppConfig.verbose_name
Human-readable name for the application, e.g. "Admin".
This attribute defaults to ``label.title()``.
.. attribute:: AppConfig.path
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
:class:`~django.apps.AppConfig` subclass. In a few situations this is
required; for instance if the app package is a `namespace package`_ with
multiple paths.
Read-only attributes
--------------------
.. attribute:: AppConfig.module
Root module for the application, e.g. ``<module 'django.contrib.admin' from
'django/contrib/admin/__init__.pyc'>``.
.. attribute:: 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.
Methods
-------
.. method:: AppConfig.get_models()
Returns an iterable of :class:`~django.db.models.Model` classes.
.. method:: AppConfig.get_model(model_name)
Returns the :class:`~django.db.models.Model` with the given
``model_name``. Raises :exc:`~exceptions.LookupError` if no such model
exists. ``model_name`` is case-insensitive.
.. method:: 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.
You cannot import models in modules that define application configuration
classes, but you can use :meth:`get_model` to access a model class by
name, like this::
def ready(self):
MyModel = self.get_model('MyModel')
.. warning::
Although you can access model classes as described above, avoid
interacting with the database in your :meth:`ready()` implementation.
This includes model methods that execute queries
(:meth:`~django.db.models.Model.save()`,
:meth:`~django.db.models.Model.delete()`, manager methods etc.), and
also raw SQL queries via ``django.db.connection``. Your
:meth:`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!
.. _namespace package:
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:
1. The namespace package actually has only a single location (i.e. is not
spread across more than one directory.)
2. The :class:`~django.apps.AppConfig` class used to configure the application
has a :attr:`~django.apps.AppConfig.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
:exc:`~django.core.exceptions.ImproperlyConfigured`.
Application registry
====================
.. data:: apps
The application registry provides the following public API. Methods that
aren't listed below are considered private and may change without notice.
.. attribute:: apps.ready
Boolean attribute that is set to ``True`` when the registry is fully
populated.
.. method:: apps.get_app_configs()
Returns an iterable of :class:`~django.apps.AppConfig` instances.
.. method:: apps.get_app_config(app_label)
Returns an :class:`~django.apps.AppConfig` for the application with the
given ``app_label``. Raises :exc:`~exceptions.LookupError` if no such
application exists.
.. method:: 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'``.
Unlike :meth:`~django.apps.apps.get_app_config`, this method can be called
safely at import time. If the registry is still being populated, it may
return ``False``, even though the app will become available later.
.. method:: apps.get_model(app_label, model_name)
Returns the :class:`~django.db.models.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 :exc:`~exceptions.LookupError` if no such application or model
exists. Raises :exc:`~exceptions.ValueError` when called with a single
argument that doesn't contain exactly one dot.