required=False allows omission of value for output. Closes #2342

docs/api-guide/fields.md

@@ -41,6 +41,8 @@ Defaults to `False`
 Normally an error will be raised if a field is not supplied during deserialization.
 Set to false if this field is not required to be present during deserialization.
 
+Setting this to `False` also allows the object attribute or dictionary key to be omitted from output when serializing the instance. If the key is not present it will simply not be included in the output representation.
+
 Defaults to `True`.
 
 ### `allow_null`

rest_framework/fields.py

@@ -288,6 +288,8 @@ 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:
+                raise SkipField()
             msg = (
                 'Got {exc_type} when attempting to get a value for field '
                 '`{field}` on serializer `{serializer}`.\nThe serializer '

rest_framework/serializers.py

@@ -419,8 +419,14 @@ class Serializer(BaseSerializer):
         fields = [field for field in self.fields.values() if not field.write_only]
 
         for field in fields:
-            attribute = field.get_attribute(instance)
+            try:
+                attribute = field.get_attribute(instance)
+            except SkipField:
+                continue
+
             if attribute is None:
+                # We skip `to_representation` for `None` values so that
+                # fields do not have to explicitly deal with that case.
                 ret[field.field_name] = None
             else:
                 ret[field.field_name] = field.to_representation(attribute)

tests/test_serializer.py

 # coding: utf-8
 from __future__ import unicode_literals
+from .utils import MockObject
 from rest_framework import serializers
 from rest_framework.compat import unicode_repr
 import pytest
@@ -216,3 +217,64 @@ class TestUnicodeRepr:
         instance = ExampleObject()
         serializer = ExampleSerializer(instance)
         repr(serializer)  # Should not error.
+
+
+class TestNotRequiredOutput:
+    def test_not_required_output_for_dict(self):
+        """
+        'required=False' should allow a dictionary key to be missing in output.
+        """
+        class ExampleSerializer(serializers.Serializer):
+            omitted = serializers.CharField(required=False)
+            included = serializers.CharField()
+
+        serializer = ExampleSerializer(data={'included': 'abc'})
+        serializer.is_valid()
+        assert serializer.data == {'included': 'abc'}
+
+    def test_not_required_output_for_object(self):
+        """
+        'required=False' should allow an object attribute to be missing in output.
+        """
+        class ExampleSerializer(serializers.Serializer):
+            omitted = serializers.CharField(required=False)
+            included = serializers.CharField()
+
+            def create(self, validated_data):
+                return MockObject(**validated_data)
+
+        serializer = ExampleSerializer(data={'included': 'abc'})
+        serializer.is_valid()
+        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 ExampleSerializer(serializers.Serializer):
+            omitted = serializers.CharField(default='abc')
+            included = serializers.CharField()
+
+        serializer = ExampleSerializer({'included': 'abc'})
+        with pytest.raises(KeyError):
+            serializer.data
+
+    def test_default_required_output_for_object(self):
+        """
+        'default="something"' should require object attribute.
+
+        We need to handle this as the field will have an implicit
+        'required=False', but it should still have a value.
+        """
+        class ExampleSerializer(serializers.Serializer):
+            omitted = serializers.CharField(default='abc')
+            included = serializers.CharField()
+
+        instance = MockObject(included='abc')
+        serializer = ExampleSerializer(instance)
+        with pytest.raises(AttributeError):
+            serializer.data