Reduced reduce() usage; refs #23796.

django.core.exceptions.ValidationError.messages() and
django.db.backends.schema.BaseDatabaseSchemaEditor._alter_field():
Replaced reduce(operator.add, ...) w/uncoupled, explicit sum()

django/core/exceptions.py

 """
 Global Django exception and warning classes.
 """
-from functools import reduce
-import operator
-
 from django.utils import six
 from django.utils.encoding import force_text
 
@@ -137,7 +134,7 @@ class ValidationError(Exception):
     @property
     def messages(self):
         if hasattr(self, 'error_dict'):
-            return reduce(operator.add, dict(self).values())
+            return sum(dict(self).values(), [])
         return list(self)
 
     def update_error_dict(self, error_dict):

django/db/backends/schema.py

 import hashlib
-import operator
 
 from django.db.backends.creation import BaseDatabaseCreation
 from django.db.backends.utils import truncate_name
@@ -7,7 +6,6 @@ from django.db.models.fields.related import ManyToManyField
 from django.db.transaction import atomic
 from django.utils.encoding import force_bytes
 from django.utils.log import getLogger
-from django.utils.six.moves import reduce
 from django.utils import six
 
 logger = getLogger('django.db.backends.schema')
@@ -609,7 +607,7 @@ class BaseDatabaseSchemaEditor(object):
             # Combine actions together if we can (e.g. postgres)
             if self.connection.features.supports_combined_alters and actions:
                 sql, params = tuple(zip(*actions))
-                actions = [(", ".join(sql), reduce(operator.add, params))]
+                actions = [(", ".join(sql), sum(params, []))]
             # Apply those actions
             for sql, params in actions:
                 self.execute(

tests/test_exceptions/test_validation_error.py

@@ -6,8 +6,8 @@ from django.core.exceptions import ValidationError
 class TestValidationError(unittest.TestCase):
     def test_messages_concatenates_error_dict_values(self):
         message_dict = {}
-        with self.assertRaises(TypeError):
-            ValidationError(message_dict).messages
+        exception = ValidationError(message_dict)
+        self.assertEqual(sorted(exception.messages), [])
         message_dict['field1'] = ['E1', 'E2']
         exception = ValidationError(message_dict)
         self.assertEqual(sorted(exception.messages), ['E1', 'E2'])