How to use class attribute in its instance method in Django?

Question

I want to use variation attribute inside to_representation function in serializers.py. When I tried to implement it an error occured. I think I still don't understand the fundamental of Python class.
Here's my serializers.py:

class MarketSerializer(serializers.ModelSerializer):
    regions = serializers.PrimaryKeyRelatedField(many=True, queryset=Region.objects.all())
    languages = serializers.PrimaryKeyRelatedField(many=True, queryset=Language.objects.all())
    variation = serializers.SerializerMethodField('get_product_variations')

    class Meta:
        model = Market
        fields = ['id', 'regions', 'languages', 'name', 'status', 'variation']
        depth = 1

    @staticmethod
    def get_product_variations(self):
        return Product.objects.filter(distributor__region__market=self).distinct().count()

    def to_representation(self, instance):
        if self.context['request'].method == 'GET':
            regions = RegionSerializer(instance.regions, many=True).data
            languages = LanguageSerializer(instance.languages, many=True).data

            data = {
                'id': instance.id,
                'regions': regions,
                'languages': languages,
                'name': instance.name,
                'status': instance.status,
                'variation': self.variation,
            }
            return data
        return Serializer.to_representation(self, instance)

'variation': self.variation gives me this error on browsable API:

AttributeError at /api/markets/
'MarketSerializer' object has no attribute 'variation'

While 'variation': MarketSerializer.variation and 'variation': self.__class__.variation give me this error:

AttributeError at /api/markets/
type object 'MarketSerializer' has no attribute 'variation'

Edit
'variation': variation gives me this error:

NameError at /api/markets/
name 'variation' is not defined

DRF does some black magic with serializer attributes. But you still call your method directly. — bruno desthuilliers
– bruno desthuilliers, Commented Jan 6, 2020 at 8:05

ruddra · Accepted Answer · 2020-01-06 09:14:57Z

1

I am not sure how this suppose to work:

@staticmethod
def get_product_variations(self):
    return Product.objects.filter(distributor__region__market=self).distinct().count()

Because, in serializer method field, the method should be like this:

def get_product_variations(self, obj):
    # not a static method
    return Product.objects.filter(distributor__region__market=obj).distinct().count()

Similarly, you can use that method in to_represent method:

def to_representation(self, instance):
  ...
  data = {
            'id': instance.id,
            'regions': regions,
            'languages': languages,
            'name': instance.name,
            'status': instance.status,
            'variation': self.get_product_variations(instance),
        }
 ...

Or better:

def to_representation(self, instance):
      data = super().to_representation(instance)
      if self.context['request'].method == 'GET':
            data['regions'] = RegionSerializer(instance.regions, many=True).data
            data['languages'] = LanguageSerializer(instance.languages, many=True).data
      return data

edited Jan 6, 2020 at 9:14

answered Jan 6, 2020 at 8:28

ruddra

52.1k7 gold badges82 silver badges105 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Nathan Over a year ago

previously get_product_variations wasn't staticmethod. Because when I tried to call it inside data = { '...': ..., } it said get_product_variations expected 2 arguments. And I only fill it with 1 argument which is instance. The auto-solution PyCharm gave me is to implement it with @staticmethod.

ruddra Over a year ago

well, did you use self.get_product_variations(instance)? get_product_variations is a object method here. BTW, I would recommend to use 2nd solution as its much cleaner

Nathan Over a year ago

Yes. I tried to use self.get_product_variations(instance) inside data = { }. It also can't be used in the class attribute variation = ... if I didn't apply the staticmethod. It gave me this error Exception Value: get_product_variations() takes 1 positional argument but 2 were given

Nathan Over a year ago

ruddra, I forgot to reply about the second solution. Thank you so much. I like it and using it now. Btw I think you mistyped data = super().to_represent(instance). Isn't it should be data = super().to_representation(instance) ? Or they're indeed different methods.

ruddra Over a year ago

No, it should be to_representation. I am fixing it now

Marlon Abeykoon · Accepted Answer · 2020-01-06 08:24:51Z

0

Since you are trying to access the outer class instance from an inner class, use factory-method to build Inner instance and pass Outer instance to it. We can not guarantee the existence of an outer class when you create an instance of the inner class

class MarketSerializer(object):
    variation = 'h'

    def MarketSerializer(self):
        return MarketSerializer.Meta(self)

    class Meta(object):
        def __init__(self, market_serializer_instance):
            self.market_serializer_instance = market_serializer_instance

        def inner_method(self):
            print(self.market_serializer_instance.variation)

edited Jan 6, 2020 at 8:24

answered Jan 6, 2020 at 8:05

Marlon Abeykoon

12.6k4 gold badges60 silver badges78 bronze badges

5 Comments

Nathan Over a year ago

I got new error: Exception Type: NameError Exception Value: name 'variation' is not defined

Nathan Over a year ago

Marlon, your answer works! Thank you. Although I was a bit confuse why there's no model and fields attributes inside class Meta(object).

Marlon Abeykoon Over a year ago

I only explained the idea. Had no time to include all your code in it. :p BTW I personally don't recommend nested classes since it does not depict any relationship, does it?

bruno desthuilliers Over a year ago

"Since you are trying to access the outer class instance from an inner class" => where have you seen this ?

bruno desthuilliers Over a year ago

"BTW I personally don't recommend nested classes" => this is part of the DRF serializers framework (and based on Django's Model and ModelForm idioms). The inner class is a namespace used by the metaclass (Python metaclass).

Collectives™ on Stack Overflow

How to use class attribute in its instance method in Django?

2 Answers 2

5 Comments

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related