Fields with 'allow_null=True' should imply a default serialization value (#5518)

* Add test for dotted source + allow_null

* Field 'allow_null' implies 'default=None'

* Field 'allow_null' provides serialization default

Author: Ryan P Kilby

rest_framework/fields.py

@@ -442,6 +442,8 @@ def get_attribute(self, instance):
         except (KeyError, AttributeError) as exc:
             if self.default is not empty:
                 return self.get_default()
+            if self.allow_null:
+                return None
             if not self.required:
                 raise SkipField()
             msg = (

tests/test_serializer.py

@@ -449,6 +449,14 @@ class Serializer(serializers.Serializer):
         assert Serializer({'nested': {'a': '3', 'b': {}}}).data == {'nested': {'a': '3', 'c': '2'}}
         assert Serializer({'nested': {'a': '3', 'b': {'c': '4'}}}).data == {'nested': {'a': '3', 'c': '4'}}
 
+    def test_default_for_allow_null(self):
+        # allow_null=True should imply default=None
+        class Serializer(serializers.Serializer):
+            foo = serializers.CharField()
+            bar = serializers.CharField(source='foo.bar', allow_null=True)
+
+        assert Serializer({'foo': None}).data == {'foo': None, 'bar': None}
+
 
 class TestCacheSerializerData:
     def test_cache_serializer_data(self):