                  model = NullableCharFieldModel
          serializer = CharFieldSerializer(data={'char': None})
          self.assertTrue(serializer.fields['char'].allow_none)
          self.assertTrue(serializer.is_valid())
          self.assertIsNone(serializer.object.char)
  
  
  class SerializerMethodFieldTest(TestCase):
      """
          Tests for the SerializerMethodField field_to_native() behavior
 
</s> remove         class CharFieldSerializer(serializers.ModelSerializer):
            class Meta:
                model = NullableCharFieldModel
 </s> add         class CharFieldSerializer(serializers.Serializer):
            char = serializers.CharField(allow_none=True, required=False) </s> remove class NullableCharFieldModel(models.Model):
    char = models.CharField(null=True, blank=True, max_length=4)


 </s> add  </s> remove                 # return None explicitly because smart_text(None) == 'None'
                # see #1834 for details
 </s> add                 # Return None explicitly because smart_text(None) == 'None'. See #1834 for details
        self.assertIsNone(serializer.object['char'])
                  model = NullableCharFieldModel
          serializer = CharFieldSerializer(data={'char': None})
          self.assertTrue(serializer.fields['char'].allow_none)
          self.assertTrue(serializer.is_valid())
        self.assertIsNone(serializer.object['char'])
  
  
  class SerializerMethodFieldTest(TestCase):
      """
          Tests for the SerializerMethodField field_to_native() behavior
keep keep keep keep replace keep keep keep keep keep
 Use Serializer instead of ModelSerializer 
tests/test_fields.py