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

This reverts commit 66757fee7e921ad4c35e0b3f80c25e026100b31c.
Discussions have led to think that this functionality does not
bring significant benefits to justify the added complexity.
Read also discussions on ticket #22734.

3 files changed, 2 insertions, 27 deletions

diff --git a/django/conf/__init__.py b/django/conf/__init__.py
index b993439018..0b20d52484 100644
--- a/django/conf/__init__.py
+++ b/django/conf/__init__.py
@@ -12,7 +12,6 @@ import time     # Needed for Windows
 
 from django.conf import global_settings
 from django.core.exceptions import ImproperlyConfigured
-from django.utils.datastructures import dict_merge
 from django.utils.functional import LazyObject, empty
 from django.utils import six
 
@@ -78,10 +77,6 @@ class BaseSettings(object):
         elif name == "ALLOWED_INCLUDE_ROOTS" and isinstance(value, six.string_types):
             raise ValueError("The ALLOWED_INCLUDE_ROOTS setting must be set "
                 "to a tuple, not a string.")
-        elif (hasattr(self, name) and name.isupper() and
-                isinstance(getattr(self, name), dict) and isinstance(value, dict)):
-            # This allows defining only a partial dict to update a global setting
-            value = dict_merge(getattr(self, name), value)
         object.__setattr__(self, name, value)
 
 
@@ -150,7 +145,7 @@ class UserSettingsHolder(BaseSettings):
         from the module specified in default_settings (if possible).
         """
         self.__dict__['_deleted'] = set()
-        self.__dict__['default_settings'] = default_settings
+        self.default_settings = default_settings
 
     def __getattr__(self, name):
         if name in self._deleted:
diff --git a/django/db/migrations/loader.py b/django/db/migrations/loader.py
index 4ba3f27808..9a3dd80d47 100644
--- a/django/db/migrations/loader.py
+++ b/django/db/migrations/loader.py
@@ -49,7 +49,7 @@ class MigrationLoader(object):
 
     @classmethod
     def migrations_module(cls, app_label):
-        if settings.MIGRATION_MODULES.get(app_label):
+        if app_label in settings.MIGRATION_MODULES:
             return settings.MIGRATION_MODULES[app_label]
         else:
             app_package_name = apps.get_app_config(app_label).name
diff --git a/django/utils/datastructures.py b/django/utils/datastructures.py
index ad8792d192..2efcd00eb0 100644
--- a/django/utils/datastructures.py
+++ b/django/utils/datastructures.py
@@ -244,26 +244,6 @@ class SortedDict(dict):
         self.keyOrder = []
 
 
-def dict_merge(a, b):
-    """
-    Utility to recursively merge two dicts, taking care not to overwrite subkeys
-    (which would happen with dict.update), but keeping existing key including
-    those from subdictionaries (optionally opted-out if a `_clear_defaults` key
-    is present).
-    Thanks Ross McFarland (https://www.xormedia.com/recursively-merge-dictionaries-in-python/)
-    """
-    if b.get('_clear_defaults'):
-        return copy.deepcopy(b)
-
-    result = copy.deepcopy(a)
-    for key, value in six.iteritems(b):
-        if key in a and isinstance(result[key], dict):
-            result[key] = dict_merge(result[key], value)
-        else:
-            result[key] = value
-    return result
-
-
 class OrderedSet(object):
     """
     A set which keeps the ordering of the inserted items.