Fixed #21555 -- Made ValidationError pickable.

Thanks trac username zanuxzan for the report and original patch.

django/core/exceptions.py

@@ -4,6 +4,7 @@ Global Django exception and warning classes.
 from functools import reduce
 import operator
 
+from django.utils import six
 from django.utils.encoding import force_text
 
 
@@ -84,10 +85,17 @@ class ValidationError(Exception):
         list or dictionary can be an actual `list` or `dict` or an instance
         of ValidationError with its `error_list` or `error_dict` attribute set.
         """
+
+        # PY2 can't pickle naive exception: http://bugs.python.org/issue1692335.
+        super(ValidationError, self).__init__(message, code, params)
+
         if isinstance(message, ValidationError):
             if hasattr(message, 'error_dict'):
                 message = message.error_dict
-            elif not hasattr(message, 'message'):
+            # PY2 has a `message` property which is always there so we can't
+            # duck-type on it. It was introduced in Python 2.5 and already
+            # deprecated in Python 2.6.
+            elif not hasattr(message, 'message' if six.PY3 else 'code'):
                 message = message.error_list
             else:
                 message, code, params = message.message, message.code, message.params

tests/validation/test_picklable.py

+import pickle
+from unittest import TestCase
+
+from django.core.exceptions import ValidationError
+
+
+class PickableValidationErrorTestCase(TestCase):
+
+    def test_validationerror_is_picklable(self):
+        original = ValidationError('a', code='something')
+        unpickled = pickle.loads(pickle.dumps(original))
+        self.assertIs(unpickled, unpickled.error_list[0])
+        self.assertEqual(original.message, unpickled.message)
+        self.assertEqual(original.code, unpickled.code)
+
+        original = ValidationError('a', code='something')
+        unpickled = pickle.loads(pickle.dumps(ValidationError(original)))
+        self.assertIs(unpickled, unpickled.error_list[0])
+        self.assertEqual(original.message, unpickled.message)
+        self.assertEqual(original.code, unpickled.code)
+
+        original = ValidationError(['a', 'b'])
+        unpickled = pickle.loads(pickle.dumps(original))
+        self.assertEqual(original.error_list[0].message, unpickled.error_list[0].message)
+        self.assertEqual(original.error_list[1].message, unpickled.error_list[1].message)
+
+        original = ValidationError(['a', 'b'])
+        unpickled = pickle.loads(pickle.dumps(ValidationError(original)))
+        self.assertEqual(original.error_list[0].message, unpickled.error_list[0].message)
+        self.assertEqual(original.error_list[1].message, unpickled.error_list[1].message)
+
+        original = ValidationError([ValidationError('a'), ValidationError('b')])
+        unpickled = pickle.loads(pickle.dumps(original))
+        self.assertIs(unpickled.args[0][0], unpickled.error_list[0])
+        self.assertEqual(original.error_list[0].message, unpickled.error_list[0].message)
+        self.assertEqual(original.error_list[1].message, unpickled.error_list[1].message)
+
+        message_dict = {'field1': ['a', 'b'], 'field2': ['c', 'd']}
+        original = ValidationError(message_dict)
+        unpickled = pickle.loads(pickle.dumps(original))
+        self.assertEqual(unpickled.message_dict, message_dict)