Delayed settings.DATABASES usage by ConnectionHandler

Refs #20474.

django/db/__init__.py

@@ -2,7 +2,6 @@ import warnings
 
 from django.conf import settings
 from django.core import signals
-from django.core.exceptions import ImproperlyConfigured
 from django.db.utils import (DEFAULT_DB_ALIAS,
     DataError, OperationalError, IntegrityError, InternalError,
     ProgrammingError, NotSupportedError, DatabaseError,
@@ -13,11 +12,7 @@ from django.utils.functional import cached_property
 __all__ = ('backend', 'connection', 'connections', 'router', 'DatabaseError',
     'IntegrityError', 'DEFAULT_DB_ALIAS')
 
-
-if settings.DATABASES and DEFAULT_DB_ALIAS not in settings.DATABASES:
-    raise ImproperlyConfigured("You must define a '%s' database" % DEFAULT_DB_ALIAS)
-
-connections = ConnectionHandler(settings.DATABASES)
+connections = ConnectionHandler()
 
 router = ConnectionRouter(settings.DATABASE_ROUTERS)
 

django/db/utils.py

@@ -6,6 +6,7 @@ import warnings
 
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
+from django.utils.functional import cached_property
 from django.utils.importlib import import_module
 from django.utils.module_loading import import_by_path
 from django.utils._os import upath
@@ -138,16 +139,27 @@ class ConnectionDoesNotExist(Exception):
 
 
 class ConnectionHandler(object):
-    def __init__(self, databases):
-        if not databases:
-            self.databases = {
+    def __init__(self, databases=None):
+        """
+        databases is an optional dictionary of database definitions (structured
+        like settings.DATABASES).
+        """
+        self._databases = databases
+        self._connections = local()
+
+    @cached_property
+    def databases(self):
+        if self._databases is None:
+            self._databases = settings.DATABASES
+        if self._databases == {}:
+            self._databases = {
                 DEFAULT_DB_ALIAS: {
                     'ENGINE': 'django.db.backends.dummy',
                 },
             }
-        else:
-            self.databases = databases
-        self._connections = local()
+        if DEFAULT_DB_ALIAS not in self._databases:
+            raise ImproperlyConfigured("You must define a '%s' database" % DEFAULT_DB_ALIAS)
+        return self._databases
 
     def ensure_defaults(self, alias):
         """