[1.7.x] Fixed #23594 -- Fixed deepcopy on ErrorList.

Thanks Troy Grosfield for the report and Tim Graham for the tests.

Backport of ec2fd02bb3 from master

django/forms/utils.py

@@ -131,6 +131,15 @@ class ErrorList(UserList, list):
             return list(error)[0]
         return force_text(error)
 
+    def __reduce_ex__(self, *args, **kwargs):
+        # The `list` reduce function returns an iterator as the fourth element
+        # that is normally used for repopulating. Since we only inherit from
+        # `list` for `isinstance` backward compatibility (Refs #17413) we
+        # nullify this iterator as it would otherwise result in duplicate
+        # entries. (Refs #23594)
+        info = super(UserList, self).__reduce_ex__(*args, **kwargs)
+        return info[:3] + (None, None)
+
 
 # Utilities for time zone support in DateTimeField et al.
 

docs/releases/1.7.1.txt

@@ -89,3 +89,5 @@ Bugfixes
 
 * Fixed ``MigrationWriter`` to handle builtin types without imports
   (:ticket:`23560`).
+
+* Fixed ``deepcopy`` on ``ErrorList`` (:ticket:`23594`).

tests/forms_tests/tests/test_util.py

@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
+import copy
+
 from django.core.exceptions import ValidationError
 from django.forms.utils import flatatt, ErrorDict, ErrorList
 from django.test import TestCase
@@ -66,3 +68,24 @@ class FormsUtilTestCase(TestCase):
                          '<ul class="errorlist"><li>nameExample of link: &lt;a href=&quot;http://www.example.com/&quot;&gt;example&lt;/a&gt;</li></ul>')
         self.assertHTMLEqual(str(ErrorDict({'name': mark_safe(example)})),
                          '<ul class="errorlist"><li>nameExample of link: <a href="http://www.example.com/">example</a></li></ul>')
+
+    def test_error_dict_copy(self):
+        e = ErrorDict()
+        e['__all__'] = ErrorList([
+            ValidationError(
+                message='message %(i)s',
+                params={'i': 1},
+            ),
+            ValidationError(
+                message='message %(i)s',
+                params={'i': 2},
+            ),
+        ])
+
+        e_copy = copy.copy(e)
+        self.assertEqual(e, e_copy)
+        self.assertEqual(e.as_data(), e_copy.as_data())
+
+        e_deepcopy = copy.deepcopy(e)
+        self.assertEqual(e, e_deepcopy)
+        self.assertEqual(e.as_data(), e_copy.as_data())