Fixes #3804, avoiding update of Meta.extra_kwargs

Added unit test to cover.

rest_framework/serializers.py

@@ -1212,7 +1212,7 @@ def get_extra_kwargs(self):
         Return a dictionary mapping field names to a dictionary of
         additional keyword arguments.
         """
-        extra_kwargs = getattr(self.Meta, 'extra_kwargs', {})
+        extra_kwargs = copy.deepcopy(getattr(self.Meta, 'extra_kwargs', {}))
 
         read_only_fields = getattr(self.Meta, 'read_only_fields', None)
         if read_only_fields is not None:

tests/test_model_serializer.py

@@ -909,3 +909,34 @@ class Meta:
         serializer = TestSerializer()
 
         assert serializer.fields['decimal_field'].max_value == 3
+
+
+class TestMetaInheritance(TestCase):
+    def test_extra_kwargs_not_altered(self):
+        class TestSerializer(serializers.ModelSerializer):
+            non_model_field = serializers.CharField()
+
+            class Meta:
+                model = OneFieldModel
+                read_only_fields = ('char_field', 'non_model_field')
+                fields = read_only_fields
+                extra_kwargs = {}
+
+        class ChildSerializer(TestSerializer):
+            class Meta(TestSerializer.Meta):
+                read_only_fields = ()
+
+        test_expected = dedent("""
+            TestSerializer():
+                char_field = CharField(read_only=True)
+                non_model_field = CharField()
+        """)
+
+        child_expected = dedent("""
+            ChildSerializer():
+                char_field = CharField(max_length=100)
+                non_model_field = CharField()
+        """)
+        self.assertEqual(unicode_repr(ChildSerializer()), child_expected)
+        self.assertEqual(unicode_repr(TestSerializer()), test_expected)
+        self.assertEqual(unicode_repr(ChildSerializer()), child_expected)