If required=False custom validation shouldn't be triggered

For optional fields (`required=False`), custom validation
(`validate_<fieldname>()`) should only be called if the field is present
in the incoming data.

rest_framework/serializers.py

@@ -286,6 +286,12 @@ class BaseSerializer(WritableField):
 
         return reverted_data
 
+    def skip_field_validation(self, field, attrs, source):
+        """
+        Checks if a field validation should be skipped.
+        """
+        return source not in attrs and (field.partial or not field.required)
+
     def perform_validation(self, attrs):
         """
         Run `validate_<fieldname>()` and `validate()` methods on the serializer
@@ -295,8 +301,9 @@ class BaseSerializer(WritableField):
                 continue
 
             source = field.source or field_name
-            if self.partial and source not in attrs:
+            if self.skip_field_validation(field_name, field, attrs):
                 continue
+
             try:
                 validate_method = getattr(self, 'validate_%s' % field_name, None)
                 if validate_method:

rest_framework/tests/test_serializer.py

@@ -562,6 +562,7 @@ class ValidationTests(TestCase):
 
 class CustomValidationTests(TestCase):
     class CommentSerializerWithFieldValidator(CommentSerializer):
+        name = serializers.CharField(max_length=255, required=False)
 
         def validate_email(self, attrs, source):
             attrs[source]
@@ -573,6 +574,12 @@ class CustomValidationTests(TestCase):
                 raise serializers.ValidationError("Test not in value")
             return attrs
 
+        def validate_name(self, attrs, source):
+            value = attrs[source]
+            if value.isupper():
+                raise serializers.ValidationError("Uppercase names not allowed")
+            return attrs
+
     def test_field_validation(self):
         data = {
             'email': 'tom@example.com',
@@ -601,6 +608,35 @@ class CustomValidationTests(TestCase):
         self.assertFalse(serializer.is_valid())
         self.assertEqual(serializer.errors, {'content': ['This field is required.']})
 
+    def test_not_required_missing_data(self):
+        """
+        Make sure that `validate_<fieldname>()` is not called if the field is not required and
+        the field is missing.
+        """
+        data = {
+            'email': 'tom@example.com',
+            'content': 'A test comment',
+            'created': datetime.datetime(2012, 1, 1)
+        }
+        serializer = self.CommentSerializerWithFieldValidator(data=data)
+        self.assertTrue(serializer.is_valid())
+        self.assertEqual(serializer.errors, {'content': ['This field is required.']})
+
+    def test_not_required_invalid_data(self):
+        """
+        Make sure that `validate_<fieldname>()` is called if the field is present, even if the
+        field is not required.
+        """
+        data = {
+            'name': 'TOM',
+            'email': 'tom@example.com',
+            'content': 'A test comment',
+            'created': datetime.datetime(2012, 1, 1)
+        }
+        serializer = self.CommentSerializerWithFieldValidator(data=data)
+        self.assertFalse(serializer.is_valid())
+        self.assertEqual(serializer.errors, {'name': ['Uppercase names not allowed']})
+
     def test_wrong_data(self):
         """
         Make sure that validate_content isn't called if the field input is wrong