How does Django start?

App loading refactor - a story

A history of ticket #3591 - the infamous app loading refactor which is coming in Django 1.7

Transcript

1. How does Django start? App loading refactor - a story
2. Start up - Django 1.0-1.6 • At some point, a model is imported • ModelBase.__new__() triggers django.db.models.loading.AppCache initialization • Reverse relations now work • Most other bits of code inspect settings.INSTALLED_APPS directly
3. #3591
4. #3591 • Opened 26th February 2007 by Joseph Kocherhans • Django 0.96 was 23rd March 2007 • “Add support for custom app label and verbose name”
5. 2007 • February 2007 - Design decision needed • September 2007 - Assigned to Adrian • December 2007 - Accepted by Jacob • Various attempted patches
6. 2007 INSTALLED_APPS = ( 'django.contrib.auth', app('mypkg.auth', 'myauth', 'My cool auth app'), )
7. 2007 INSTALLED_APPS = ( 'django.contrib.auth', app('mypkg.auth', { 'app_label': 'myauth', 'verbose_name': 'My cool auth app' }), )
8. PyCon 2008 • “The state of Django” - Adrian Holovaty • Serious attempt at sprints • Goals were: • Change (verbose) name • Change db_prefix • Multiple instances of app
9. PyCon 2008 • It didn’t work.
10. PyCon 2008 • Scope became very much about rewriting how django handles applications, not just adding a piece of admin functionality • class InstalledApps() • Overloading of app_label • Clear requirement for App.path
11. 2009 • Nothing
12. 2010 • February 2010 - Design decision needed • Proposed for GSOC 2010 • “Medium complexity”
13. GSOC 2010 • Arthur Koziel (mentored by Jannis) • Goals: • Multiple instances of apps • A way to avoid name clashes • Internationalisation of application names
14. GSOC 2010 • September 2010 - “Fixed on a branch”
15. 2011 • Ongoing work by Arthur and Jannis • Some unpopular implementation details • Metaclasses for Apps • Reloadable app cache
16. 2012 • Preston Holmes brings patch up to speed • Lots of conflicts • Lots of failing tests • Died out late 2012
17. 2013 • Migrations needed an app cache
19. 2013 • Still assigned to Adrian…
20. December 2013 • Aymeric takes a holiday
21. December 2013
22. December 2013 • Timezones • Python 3 • Database transactions • Django Debug Toolbar 1.0 • “He also did an multi-day in-depth review and extended discussion as a technical reviewer of Two Scoops of Django 1.6” - Danny Roy Greenfeld
23. December 2013 • Rethinking the approach • Minimal change • Just get some of it working • Old patches abandoned
24. December 2013 • class Apps() • class AppConfig() • No models.py required • Customisable verbose name • Start up code • Customisable label and filesystem path
25. December 2013 • Merged!
26. December 2013 X
27. Django 1.7 • What does it mean for you? • Existing projects and libraries may break • Hopefully, it’s just because you did it wrong
28. Django 1.7 • 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”
29. Django 1.7 • Module level code which requires the apps to be loaded will break (in models.py,) • Likely ugettext() - use ugettext_lazy() • Code relying on import order of models.py files may break • Code relying on “for app in INSTALLED_APPS” will break
30. Django 1.7 • Internationalised names in contrib apps • No need for admin.autodiscover() in urls
31. Use cases • Registering signals in my project specific app • Making my third party application have an internationalised name in the admin • Customising a third party application
32. Register signals # blog.apps ! 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)
33. Register signals # myproject.settings ! INSTALLED_APPS = ( … 'blog.apps.BlogConfig', … )
34. Third party apps # ponies.apps ! from django import apps from … import ugettext_lazy as _ ! class PonyConfig(apps.AppConfig): name = 'ponies' verbose_name = _('the stables') ! # ponies.__init__ ! default_app_config = 'ponies.apps.PonyConfig'
35. Customising third party # myproject.apps ! from django.db.models.signals import post_save from ponies.apps import PonyConfig ! class MyPonyConfig(PonyConfig): name = 'ponies' def ready(self): from ponies.models import Pony from pony_util.signals import flush_cache post_save.connect(flush_cache, sender=Pony)
36. Customising third party # myproject.settings ! INSTALLED_APPS = ( … 'myproject.apps.MyPonyConfig', … )
37. The future? • AppAdmin - more advanced API for customising appearance of apps in the admin • App specific settings
38. The future? (+ponies) • Setup time customisation of models • Add fields • Change field lengths • Add indexes
39. Questions? @mjtamlyn github.com/mjtamlyn