Django 3.2 automatic AppConfig discovery breaks projects working in 3.1.8

[Manel Clos]

The described method of overriding the default admin site (​https://docs.djangoproject.com/en/3.2/ref/contrib/admin/#overriding-the-default-admin-site), is not compatible with automatic AppConfig discovery, if the app (myproject, or myapps) is already in INSTALLED_APPS.

How to reproduce

• pip install Django==3.2
• django-admin startproject myproject
• python manage.py migrate
• python manage.py createsuperuser
• Put a template in myproject/templates/admin/base_site.html

  {% extends 'admin/base_site.html' %}
  
  {% block branding %}
  Template test
  {% endblock %}
  

• Add myproject to INSTALLED_APPS

  INSTALLED_APPS = [
      'myproject',
  
      'django.contrib.admin',
      'django.contrib.auth',
      'django.contrib.contenttypes',
      'django.contrib.sessions',
      'django.contrib.messages',
      'django.contrib.staticfiles',
  ]
  

• python manage.py runserver works, I can access the admin site and see the changes in templates
• Follow the guide to override the default admin site, ​https://docs.djangoproject.com/en/3.2/ref/contrib/admin/#overriding-the-default-admin-site, this is, create myproject/admin.py, myproject/apps.py and modify settings.py.

  # myproject/admin.py
  from django.contrib import admin
  
  class MyAdminSite(admin.AdminSite):
      index_title = 'Test'
  
  
  # myproject/apps.py
  from django.contrib.admin.apps import AdminConfig
  
  class MyAdminConfig(AdminConfig):
      default_site = 'myproject.admin.MyAdminSite'
  
  
  # myproject/settings.py
  INSTALLED_APPS = [
      'myproject',
  
      'myproject.apps.MyAdminConfig',  # 'django.contrib.admin',
      'django.contrib.auth',
      'django.contrib.contenttypes',
      'django.contrib.sessions',
      'django.contrib.messages',
      'django.contrib.staticfiles',
  ]
  

• python manage.py runserver

Actual result
RuntimeError: 'myproject.apps' declares more than one default AppConfig: 'AdminConfig', 'MyAdminConfig'.`

This a regression

• pip install Django==3.1.8
• python manage.py runserver
• All ok, both templates and AdminConfig working at the same time

Workaroud

• use app-admin.py instead of apps.py when following the documentation to override the default admin site

  from django.contrib.admin.apps import AdminConfig
  
  class MyAdminConfig(AdminConfig):
      default_site = 'myproject.admin.MyAdminSite'
  

• adjust settings.py: 'myproject.apps-admin.MyAdminConfig' in INSTALLED_APPS

  INSTALLED_APPS = [
      'myproject',
  
      'myproject.apps-admin.MyAdminConfig',  # 'django.contrib.admin',
      'django.contrib.auth',
      'django.contrib.contenttypes',
      'django.contrib.sessions',
      'django.contrib.messages',
      'django.contrib.staticfiles',
  ]
  

• works with Django 3.2 now!