  
          return instance
  
      def update(self, instance, validated_attrs):
          for attr, value in validated_attrs.items():
              setattr(instance, attr, value)
          instance.save()
          return instance
  
      def get_unique_together_validators(self):
 
</s>
        assert not any(
            isinstance(field, BaseSerializer) and not field.read_only
            for field in self.fields.values()
        ), (
            'The `.update()` method does not suport nested writable fields '
            'by default. Write an explicit `.update()` method for serializer '
            '%s, or set `read_only=True` on nested serializer fields.' %
            self.__class__.__name__
        )

  
          return instance
  
      def update(self, instance, validated_attrs):
        assert not any(
            isinstance(field, BaseSerializer) and not field.read_only
            for field in self.fields.values()
        ), (
            'The `.update()` method does not suport nested writable fields '
            'by default. Write an explicit `.update()` method for serializer '
            '%s, or set `read_only=True` on nested serializer fields.' %
            self.__class__.__name__
        )

          for attr, value in validated_attrs.items():
              setattr(instance, attr, value)
          instance.save()
          return instance
  
      def get_unique_together_validators(self):
keep keep keep add keep keep keep keep keep keep
 Stricter checking for failure cases.
rest_framework/serializers.py