Class reference inside class scope in python

Question

Some Context

I have the following django/python snippet:

from rest_framework import serializers
from .models import Profile, Task

class Serializable():
    types = {}
    def __init__(self, objectid):
        self.object = self.types[objectid][0]
        self.serializer = self.types[objectid][1]

    def serialized(self):
        instances = self.object.objects.all()
        serialized = self.serializer(instances, many=True)
        return serialized


class ProfileSerializer(serializers.ModelSerializer):
    class Meta:
        oid = 'profile'
        model = Profile
        fields = ['login', 'status']
        Serializable.types[oid] = [model, <class-reference>]

class TaskSerializer(serializers.ModelSerializer):
    class Meta:
        oid = 'task'
        model = Task
        fields = ['description', 'date', 'owner']
        Serializable.types[oid] = [model, <class-reference>]

I am using Django with the rest_framework library installed. One of the interesting features I am using is ModelSerializers (ModelSerializers Documentation), which save quite a lot of code repetition. I want Serializable.types variable to be populated on runtime (when all the serializer classes are declared). The whole point of this is that I will not have to update my views whens a new type of model is included. For example, I would print the json representation of my model instances like this:

class QueryObject(APIView):
    permission_classes = (AllowAny,)

    def get(self, request, *args, **kwargs):
        oid = request.GET['oid']
        serializable= Serializable(oid)
        json = serializable.serialized
        return JsonResponse(json)

The Problem

The major problem is in the last line of each Serializer class.

Serializable.types[oid] = [model, <class-reference>]

I've tried putting the name of the class, ProfileSerializer for example, to no avail. I've tried doing the same outside of the Meta class, such as:

class ProfileSerializer(serializers.ModelSerializer):
    class Meta:
        oid = 'profile'
        model = Profile
        fields = ['login', 'status']

    Serializable.types[Meta.oid] = [Meta.model, ProfileSerializer]

also not successful. Not sure what else to do, which is why I'm hoping the community can help me on this one.

Philip Adler · Accepted Answer · 2017-06-18 16:33:06Z

2

This is actually a case for defining a metaclass.

I've never actually found a source of information which gives a complete, clear and satisfactory explanation as to what metaclasses are or how they work. I will try to enhance this answer with such information if required but for the time being I am going to stick to a solution for your present problem. I am assuming python 3.

Define an additional class, thus:

class ModelSerializerMeta(serializers.SerializerMetaclass):

    def __init__(cls, class_name, base_classes, attributes):

        super(ModelSerialiserMeta, cls).__init__(class_name, base_classes, attributes)
        Serializer.types[cls.Meta.oid] = [cls.Meta.model, cls]

Then use this as the metaclass of your Serializers, e.g.

class ProfileSerializer(serializers.ModelSerializer, metaclass=ModelSerializerMeta):

    class Meta:
        oid = 'profile'
        model = Profile
        fields = ['login', 'status']

Better yet, create some superclass for all your model serializers, assign the metaclass there, make all of your serializers inherit from that superclass which will then use the metaclass throughout.

edited Jun 18, 2017 at 16:33

answered Jun 18, 2017 at 15:49

Philip Adler

2,22820 silver badges27 bronze badges

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

5 Comments

jhc Over a year ago

Can you explain to me how my Serializable class will eventually be called? I can see that it's in the ModelSerializerMeta (cls.types), but where is cls actually defined?

Philip Adler Over a year ago

cls is defined by the Python interpreter. The one sentence reduction is that the ModelSerializerMeta code is executed when the interpreter encounters your class definition, which is then passed to the metaclass to manage the class creation. If that is insufficient, I strongly recommend googling around for python metaclasses, as there's a lot of writing out there, but I don't really know how to explain it any better.

Philip Adler Over a year ago

You would call your Serializable class in the same way you would normally.

jhc Over a year ago

@PhillipAdler, I used your solution, but changed cls.types[cls.Meta.oid] = [cls.Meta.model, cls] to Serializer.types[cls.Meta.oid] = [cls.Meta.model, cls], and it worked perfectly. Feel free to edit your answer, so I can put it as the solution.

Philip Adler Over a year ago

@jhc whoops! That was a silly mistake. Anyway, edited to fit.

nyanpasu64 · Accepted Answer · 2018-12-05 07:30:22Z

1

Metaclasses are definitely the right answer unless your code can require python >= 3.6. Starting with 3.6 there is a new feature called the __init_subclass__ hook.

So you can do something like

class foo:

    @classmethod
    def __init_subclass__(cls, *args, **kwargs):
        Serializers.register_class(cls)

Whenever a child of Foo is defined, the __init_subclass__ method on Foo will be called, passing in the child class reference as cls.

edited Dec 5, 2018 at 7:30

nyanpasu64

3,1852 gold badges28 silver badges34 bronze badges

answered Jun 18, 2017 at 16:02

Sam Hartman

6,5393 gold badges27 silver badges45 bronze badges

2 Comments

Philip Adler Over a year ago

I am so looking forward to that feature.

jhc Over a year ago

@SamHartman like I asked Philip, how does Serializable get called here?

Collectives™ on Stack Overflow

Class reference inside class scope in python

Some Context

The Problem

2 Answers 2

5 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

Some Context

The Problem

2 Answers 2

5 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related