============
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`.

Django uses the default :class:`~django.apps.AppConfig` class when
:setting:`INSTALLED_APPS` simply contains the dotted path to an application
module.

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/app.py

    from django.apps import AppConfig

    class RockNRollConfig(AppConfig):
        name = 'rock_n_roll'
        verbose_name = "Rock ’n’ roll"

You would then tell your users to add ``'rock_n_roll.app.RockNRollConfig'`` to
their :setting:`INSTALLED_APPS`.

The recommended convention is to put the configuration class in a submodule of
the application called ``app``. 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.

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.app 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()``.

Read-only attributes
--------------------

.. attribute:: AppConfig.path

    Filesystem path to the application directory, e.g.
    ``'/usr/lib/python2.7/dist-packages/django/contrib/admin'``.

    It may be ``None`` if the application isn't stored in a directory, for
    instance if it's loaded from an egg.

.. 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.

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.

.. method:: apps.ready()

    Returns ``True`` if 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.has_app(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``. Raises :exc:`~exceptions.LookupError` if no such
    application or model exists. ``model_name`` is case-insensitive.
