[1.7.x] Fixed #22915 -- Document backward incompatible changes in the ValidationError constructor.

This patch also fixes update_error_dict to better handle the use case described
in this ticket, previously the type of the provided container could be lost in
some conditions.

Thanks Russell Keith-Magee for the report and Tim Graham for review.

Backport of eb7df6b8d7 from master

1 files changed, 3 insertions, 6 deletions

diff --git a/django/core/exceptions.py b/django/core/exceptions.py
index 552b0067c3..28e712e0a5 100644
--- a/django/core/exceptions.py
+++ b/django/core/exceptions.py
@@ -142,13 +142,10 @@ class ValidationError(Exception):
 
     def update_error_dict(self, error_dict):
         if hasattr(self, 'error_dict'):
-            if error_dict:
-                for field, errors in self.error_dict.items():
-                    error_dict.setdefault(field, []).extend(errors)
-            else:
-                error_dict = self.error_dict
+            for field, error_list in self.error_dict.items():
+                error_dict.setdefault(field, []).extend(error_list)
         else:
-            error_dict[NON_FIELD_ERRORS] = self.error_list
+            error_dict.setdefault(NON_FIELD_ERRORS, []).extend(self.error_list)
         return error_dict
 
     def __iter__(self):