Creating app configuration

When developing a website with Django, you create one module for the project itself and then, multiple Python modules called applications or apps that combine the different modular functionalities and usually consist of models, views, forms, URL configurations, management commands, migrations, signals, tests, and so on. The Django framework has application registry, where all apps and models are collected and later used for configuration and introspection. Since Django 1.7, meta information about apps can be saved in the AppConfig instance for each used app. Let's create a sample magazine app to take a look at how to use the app configuration there.

Getting ready

Either create your Django app manually or using this command in your virtual environment (learn how to use virtual environments in the Working with a virtual environment recipe), as follows:

(myproject_env)$ django-admin.py startapp magazine

Add some NewsArticle model to models.py, create administration for the model in admin.py, and put "magazine" in INSTALLED_APPS in the settings. If you are not yet familiar with these tasks, study the official Django tutorial at https://docs.djangoproject.com/en/1.8/intro/tutorial01/.

How to do it…

Follow these steps to create and use the app configuration:

1. First of all, create the apps.py file and put this content in it, as follows:

   # magazine/apps.py
   # -*- coding: UTF-8 -*-
   from __future__ import unicode_literals
   from django.apps import AppConfig
   from django.utils.translation import ugettext_lazy as _
   
   class MagazineAppConfig(AppConfig):
       name = "magazine"
       verbose_name = _("Magazine")
   
       def ready(self):
           from . import signals

2. Then, edit the __init__.py file of the app and put the following content:

   # magazine/__init__.py
   # -*- coding: UTF-8 -*-
   from __future__ import unicode_literals
   default_app_config = "magazine.apps.MagazineAppConfig"

3. Lastly, let's create a signals.py file and add some signal handlers there:

   # magazine/signals.py
   # -*- coding: UTF-8 -*-
   from __future__ import unicode_literals
   from django.db.models.signals import post_save, post_delete
   from django.dispatch import receiver
   from django.conf import settings
   from .models import NewsArticle
   
   @receiver(post_save, sender=NewsArticle)
   def news_save_handler(sender, **kwargs):
       if settings.DEBUG:
           print("%s saved." % kwargs['instance'])
   
   @receiver(post_delete, sender=NewsArticle)
   def news_delete_handler(sender, **kwargs):
       if settings.DEBUG:
           print("%s deleted." % kwargs['instance'])

How it works…

When you run an HTTP server or invoke a management command, django.setup() is called. It loads the settings, sets up logging, and initializes app registry. The app registry is initialized in three steps, as shown in the following:

• Django imports the configurations for each item from INSTALLED_APPS in the settings. These items can point to app names or configuration directly, for example,"magazine" or "magazine.apps.NewsAppConfig".
• Django tries to import models.py from each app in INSTALLED_APPS and collect all the models.
• Finally, Django runs the ready() method for each app configuration. This method is a correct place to register signal handlers, if you have any. The ready() method is optional.
• In our example, the MagazineAppConfig class sets the configuration for the magazine app. The name parameter defines the name of the current app. The verbose_name parameter is used in the Django model administration, where models are presented and grouped by apps. The ready() method imports and activates the signal handlers that, when in DEBUG mode, print in the terminal that a NewsArticle was saved or deleted.

There is more…

After calling django.setup(), you can load the app configurations and models from the registry as follows:

>>> from django.apps import apps as django_apps
>>> magazine_app_config = django_apps.get_app_config("magazine")
>>> magazine_app_config
<MagazineAppConfig: magazine>
>>> magazine_app_config.models_module
<module 'magazine.models' from 'magazine/models.pyc'>
NewsArticle = django_apps.get_model("magazine", "NewsArticle")

You can read more about app configuration in the official Django documentation at https://docs.djangoproject.com/en/1.8/ref/applications/

See also

• The Working with a virtual environment recipe
• The Defining overwritable app settings recipe
• Chapter 6, Model Administration