Allow required false and default (#4692)

* Default value will now be used when serializing if key or attribute is missing.

docs/api-guide/fields.md

@@ -55,6 +55,8 @@ The `default` is not applied during partial update operations. In the partial up
 
 May be set to a function or other callable, in which case the value will be evaluated each time it is used. When called, it will receive no arguments. If the callable has a `set_context` method, that will be called each time before getting the value with the field instance as only argument. This works the same way as for [validators](validators.md#using-set_context).
 
+When serializing the instance, default will be used if the the object attribute or dictionary key is not present in the instance.
+
 Note that setting a `default` value implies that the field is not required. Including both the `default` and `required` keyword arguments is invalid and will raise an error.
 
 ### `source`

rest_framework/fields.py

@@ -443,7 +443,9 @@ class Field(object):
         try:
             return get_attribute(instance, self.source_attrs)
         except (KeyError, AttributeError) as exc:
-            if not self.required and self.default is empty:
+            if self.default is not empty:
+                return self.get_default()
+            if not self.required:
                 raise SkipField()
             msg = (
                 'Got {exc_type} when attempting to get a value for field '

tests/test_serializer.py

@@ -372,36 +372,44 @@ class TestNotRequiredOutput:
         serializer.save()
         assert serializer.data == {'included': 'abc'}
 
-    def test_default_required_output_for_dict(self):
-        """
-        'default="something"' should require dictionary key.
 
-        We need to handle this as the field will have an implicit
-        'required=False', but it should still have a value.
-        """
+class TestDefaultOutput:
+    def setup(self):
         class ExampleSerializer(serializers.Serializer):
-            omitted = serializers.CharField(default='abc')
-            included = serializers.CharField()
+            has_default = serializers.CharField(default='x')
+            has_default_callable = serializers.CharField(default=lambda: 'y')
+            no_default = serializers.CharField()
+        self.Serializer = ExampleSerializer
 
-        serializer = ExampleSerializer({'included': 'abc'})
-        with pytest.raises(KeyError):
-            serializer.data
+    def test_default_used_for_dict(self):
+        """
+        'default="something"' should be used if dictionary key is missing from input.
+        """
+        serializer = self.Serializer({'no_default': 'abc'})
+        assert serializer.data == {'has_default': 'x', 'has_default_callable': 'y', 'no_default': 'abc'}
 
-    def test_default_required_output_for_object(self):
+    def test_default_used_for_object(self):
         """
-        'default="something"' should require object attribute.
+        'default="something"' should be used if object attribute is missing from input.
+        """
+        instance = MockObject(no_default='abc')
+        serializer = self.Serializer(instance)
+        assert serializer.data == {'has_default': 'x', 'has_default_callable': 'y', 'no_default': 'abc'}
 
-        We need to handle this as the field will have an implicit
-        'required=False', but it should still have a value.
+    def test_default_not_used_when_in_dict(self):
         """
-        class ExampleSerializer(serializers.Serializer):
-            omitted = serializers.CharField(default='abc')
-            included = serializers.CharField()
+        'default="something"' should not be used if dictionary key is present in input.
+        """
+        serializer = self.Serializer({'has_default': 'def', 'has_default_callable': 'ghi', 'no_default': 'abc'})
+        assert serializer.data == {'has_default': 'def', 'has_default_callable': 'ghi', 'no_default': 'abc'}
 
-        instance = MockObject(included='abc')
-        serializer = ExampleSerializer(instance)
-        with pytest.raises(AttributeError):
-            serializer.data
+    def test_default_not_used_when_in_object(self):
+        """
+        'default="something"' should not be used if object attribute is present in input.
+        """
+        instance = MockObject(has_default='def', has_default_callable='ghi', no_default='abc')
+        serializer = self.Serializer(instance)
+        assert serializer.data == {'has_default': 'def', 'has_default_callable': 'ghi', 'no_default': 'abc'}
 
 
 class TestCacheSerializerData: