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.

1 files changed, 20 insertions, 0 deletions

diff --git a/django/utils/datastructures.py b/django/utils/datastructures.py
index 2efcd00eb0..ad8792d192 100644
--- a/django/utils/datastructures.py
+++ b/django/utils/datastructures.py
@@ -244,6 +244,26 @@ 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.