Fixed #23384 -- Allowed overriding part of a dictionary-type setting

This change is needed for upcoming changes where settings might be
grouped in a parent dictionary.
Thanks Tim Graham for the review.

tests/cache/tests.py

@@ -522,6 +522,7 @@ class BaseCacheTests(object):
     def _perform_cull_test(self, cull_cache, initial_count, final_count):
         # Create initial cache key entries. This will overflow the cache,
         # causing a cull.
+        cull_cache.clear()
         for i in range(1, initial_count):
             cull_cache.set('cull%d' % i, 'value', 1000)
         count = 0
@@ -918,7 +919,10 @@ class DBCacheTests(BaseCacheTests, TransactionTestCase):
             stdout=stdout
         )
         self.assertEqual(stdout.getvalue(),
-            "Cache table 'test cache table' already exists.\n" * len(settings.CACHES))
+            "Cache table 'test cache table' already exists.\n" * len([
+                k for k, v in settings.CACHES.items()
+                if v['BACKEND']=='django.core.cache.backends.db.DatabaseCache'])
+        )
 
     def test_createcachetable_with_table_argument(self):
         """

tests/migrations/test_executor.py

@@ -196,7 +196,7 @@ class ExecutorTests(MigrationTestBase):
     @override_settings(
         MIGRATION_MODULES={
             "migrations": "migrations.test_migrations_custom_user",
-            "django.contrib.auth": "django.contrib.auth.migrations",
+            "auth": "django.contrib.auth.migrations",
         },
         AUTH_USER_MODEL="migrations.Author",
     )

tests/migrations/test_loader.py

@@ -81,7 +81,10 @@ class LoaderTests(TestCase):
         # Ensure we've included unmigrated apps in there too
         self.assertIn("basic", project_state.real_apps)
 
-    @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_unmigdep"})
+    @override_settings(MIGRATION_MODULES={
+        "_clear_defaults": True,
+        "migrations": "migrations.test_migrations_unmigdep"
+    })
     def test_load_unmigrated_dependency(self):
         """
         Makes sure the loader can load migrations with a dependency on an unmigrated app.

tests/settings_tests/tests.py

@@ -273,6 +273,63 @@ class SettingsTests(TestCase):
         self.assertRaises(ValueError, setattr, settings,
             'ALLOWED_INCLUDE_ROOTS', '/var/www/ssi/')
 
+    def test_dict_setting(self):
+        """
+        Test that dictionary-type settings can be "complemented", that is existing
+        setting keys/values are not overriden by user settings, but merged into the
+        existing dict.
+        """
+        s = LazySettings()  # Start with fresh settings from global_settings.py
+        # Simply overwriting the key
+        s.configure(CACHES={'default': {'BACKEND': 'django.core.cache.backends.dummy.DummyCache'}})
+        self.assertEqual(s.CACHES['default']['BACKEND'],
+                         'django.core.cache.backends.dummy.DummyCache')
+
+        s = LazySettings()
+        # More complex overwriting
+        s.configure(CACHES={
+            'default': {'LOCATION': 'unique-snowflake'},
+            'temp': {'BACKEND': 'django.core.cache.backends.dummy.DummyCache'}
+        })
+        self.assertDictEqual(s.CACHES, {
+            'default': {
+                'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
+                'LOCATION': 'unique-snowflake'
+            },
+            'temp': {
+                'BACKEND': 'django.core.cache.backends.dummy.DummyCache',
+            }
+        })
+
+    def test_dict_setting_clear_defaults(self):
+        """
+        Test the ability to deactivate the merge feature of dictionary settings.
+        """
+        s = LazySettings()
+        s.configure(CACHES={
+            '_clear_defaults': True,
+            'temp': {'BACKEND': 'django.core.cache.backends.dummy.DummyCache'}
+        })
+        self.assertDictEqual(s.CACHES, {
+            '_clear_defaults': True,
+            'temp': {'BACKEND': 'django.core.cache.backends.dummy.DummyCache'}
+        })
+
+        # Also work on a subkey
+        s = LazySettings()
+        s.configure(CACHES={
+            'default': {
+                '_clear_defaults': True,
+                'LOCATION': 'unique-snowflake',
+            }
+        })
+        self.assertDictEqual(s.CACHES, {
+            'default': {
+                '_clear_defaults': True,
+                'LOCATION': 'unique-snowflake',
+            }
+        })
+
 
 class TestComplexSettingOverride(TestCase):
     def setUp(self):