How does Django start?

• Marc Tamlyn
• @mjtamlyn
• github.com/mjtamlyn
• slides https://speakerdeck.com/mjtamlyn/how-does-django-start

The old days

(up to 1.6)

• at some point a model is imported
• metaclass: ModelBase.__new__() triggers django.db.models.loading.AppCache init
• reverse relations now work
• Most other bits of code inspect settings.INSTALLED_APPS directly

Problem is “at some point” - sometimes deploys can fail unpredictably

The long and winding road

#3591 bug

• opened Feb 2007 (django 0.96 released March 2007)
• “Add support for custom app label and verbose name”
• Feb 2007 - design decision needed
• Sept 2007 - assigned to Adrian Holovaty
• Dec 2007 - accepted by Jacob
• various attempted patches
• PyCon 2008 - “The state of Django” - Adrian Holovaty - sprints - goals were similar - scope became about rewriting how django handles applications - class InstalledApps()
• app_label is used in lots of places - which ones should change?
• clear requirement for App.path
• 2009 - basically nothing happened
• Feb 2010 - revert to Design decision needed
• Google SoC 2010 - “medium complexity” - multiple instances of apps, avoid names clashes, i18n of app names
• Sept 2010 - “Fixed on a branch”
• 2011 - ongoing work - unpopular details (Metaclasses for Apps, reloadable app cache)
• 2012 - patch brought up to speed, conflicts, broken tests, effort died
• 2013 - migrations needed an app cache
• Dec 2013 - Aymeric takes a holiday - timezones, python 3, db txn, debug toolbar, 2 scoops review
• and rethink the approach, minimal change

Now

• class Apps()
• class AppConfig()
• no models.py required
• customisable verbose name
• reliable place for start up code (not just “at the bottom of models.py”)
• customise label ...
• merged! -> Django 1.7

What does it mean for you?

• may break existing projects and libraries
• hopefully it’s because you did it wrong ...
• now have much stricter startup sequence
• django.setup() configures settings, imports all applications
• trying to use the app registry before it’s loaded will error “Apps not loaded”

The following will break

• modules level code which requires the apps to be loaded will break - eg ugettext() - use ugettext_lazy() - ugettext() requires apps to be loaded
• code relying on import order of models.py files may break
• code relying on for app in INSTALLED_APPS

Good stuff

• i18n names in contrib apps
• no need for admin.autodiscover() in urls (as we have a consistent place for start up)

Use cases

• registering signals in my project specific app
• making my 3rd party app have an i18n name in admin
• customise a 3rd party app

from django import apps
from django.db.models.signals import post_save

class BlogApp(apps.AppConfig):
    name = 'blog'

    def ready(self):
        from .models import Post
        from .signals import flush_cache
        post_save.connect(flush_cache, sender=Post)

# ...

INSTALLED_APPS = (
    # ...
    'blog.app.BlogApp',
    # ...
)

3rd party app with i18n verbose name

from django import apps

class PonyConfig(apps.AppConfig):
    name = 'ponies'
    verbose_name = _('mor ponies')

# ...

INSTALLED_APPS = (
    # ...
    'ponies.apps.PonyConfig',
    # ...
)

customise a 3rd party app

class MyPonyConfig(PonyConfig):

    def ready(self):
        # stuff


INSTALLED_APPS = (
    # ...
    'myproject.apps.MyPonyConfig',
    # ...
)

Note this makes it a little tricky to work out whether a model has been loaded.

Future

• AppAdmin - more advanced API for custom appearance of apps in admin
• App specific settings
• set up time customise of models - add fields, change field lengths, add indexes