Django Styleguide

Django styleguide used in HackSoft projects.

Expect often updates as we discuss & decide upon different things.

Table of contents:

• Overview
• Services
• Selectors
• APIs & Serializers
  • An example list API
  • An example detail API
  • An example create API
  • An example update API
  • Nested serializers
• Exception Handling
  • Raising Exceptions in Services
  • Handle Exceptions in APIs
• Inspiration

Overview

In Django, business logic should live in:

• Model properties (with some exceptions).
• Model clean method for additional validations (with some exceptions).
• Services - functions, that take care of code written to the database.
• Selectors - functions, that take care of code taken from the database.

In Django, business logic should not live in:

• APIs and Views.
• Serializers and Forms.
• Form tags.
• Model save method.

Model properties vs selectors:

• If the model property spans multiple relations, it should better be a selector.
• If a model property, added to some list API, will cause N + 1 problem that cannot be easily solved with select_related, it should better be a selector.

Services

A service is a simple function that:

• Lives in your_app/services.py module
• Takes keyword-only arguments
• Is type-annotated (even if you are not using mypy at the moment)
• Works mostly with models & other services and selectors
• Does business logic - from simple model creation to complex cross-cutting concerns, to calling external services & tasks.

An example service that creates an user:

def create_user(
    *,
    email: str,
    name: str
) -> User:
    user = User(email=email)
    user.full_clean()
    user.save()

    create_profile(user=user, name=name)
    send_confirmation_email(user=user)

    return user

As you can see, this service calls 2 other services - create_profile and send_confirmation_email

Selectors

A selector is a simple function that:

• Lives in your_app/selectors.py module
• Takes keyword-only arguments
• Is type-annotated (even if you are not using mypy at the moment)
• Works mostly with models & other services and selectors
• Does business logic around fetching data from your database

An example selector that list users from the database:

def get_users(*, fetched_by: User) -> Iterable[User]:
    user_ids = get_visible_users_for(user=fetched_by)

    query = Q(id__in=user_ids)

    return User.objects.filter(query)

As you can see, get_visible_users_for is another selector.

APIs & Serializers

When using services & selectors, all of your APIs should look simple & the same.

General rules for an API is:

• Do 1 API per operation. For CRUD on a model, this means 4 APIs.
• Use the most simple APIView or GenericAPIView
• Use services / selectors & don't do business logic in your API.
• Use serializers for fetching objects from params - passed either via GET or POST
• Serializer should be nested in the API and be named either InputSerializer or OutputSerializer
  • OutputSerializer can subclass ModelSerializer, if needed.
  • InputSerializer should always be a plain Serializer
  • Reuse serializers as little as possible
  • If you need a nested serializer, use the inline_serializer util

An example list API

class CourseListApi(SomeAuthenticationMixin, APIView):
    class OutputSerializer(serializers.ModelSerializer):
        class Meta:
            model = Course
            fields = ('id', 'name', 'start_date', 'end_date')

    def get(self, request):
        courses = get_courses()

        data = self.OutputSerializer(courses, many=True)

        return Response(data)

An example detail API

class CourseDetailApi(SomeAuthenticationMixin, APIView):
    class OutputSerializer(serializers.ModelSerializer):
        class Meta:
            model = Course
            fields = ('id', 'name', 'start_date', 'end_date')

    def get(self, request, course_id):
        course = get_course(id=course_id)

        data = self.OutputSerializer(course)

        return Response(data)

An example create API

class CourseCreateApi(SomeAuthenticationMixin, APIView):
    class InputSerializer(serializers.Serializer):
        name = serializers.CharField()
        start_date = serializers.DateField()
        end_date = serializers.DateField()

    def post(self, request):
        serializer = self.InputSerializer(data=request.data)
        serializer.is_valid(raise_exception=True)

        create_course(**serializer.validated_data)

        return Response(status=status.HTTP_201_CREATED)

An example update API

class CourseUpdateApi(SomeAuthenticationMixin, APIView):
    class InputSerializer(serializers.Serializer):
        name = serializers.CharField(required=False)
        start_date = serializers.DateField(required=False)
        end_date = serializers.DateField(required=False)

    def post(self, request, course_id):
        serializer = self.InputSerializer(data=request.data)
        serializer.is_valid(raise_exception=True)

        update_course(course_id=course_id, **serializer.validated_data)

        return Response(status=status.HTTP_200_OK)

Nested serializers

In case you need to use a nested serializer, you can do the following thing:

class Serializer(serializers.Serializer):
    weeks = inline_serializer(many=True, fields={
        'id': serializers.IntegerField(),
        'number': serializers.IntegerField(),
    })

The implementation of inline_serializer can be found in utils.py in this repo.

Exception Handling

Raising Exceptions in Services / Selectors

Now we have separation between our HTTP interface & the core logic of our application.

In order to keep this separation of concerns, our services and selectors must not use the rest_framework.exception classes because they are bounded with HTTP status codes.

Our services and selectors must use one of:

• Python built-in exceptions
• Exceptions from django.core.exceptions
• Custom exceptions, inheriting from the ones above.

Here is a good example of service that preforms some validation and raises django.core.exceptions.ValidationError:

from django.core.exceptions import ValidationError

def create_topic(*, name: str, course: Course) -> Topic:
    if course.end_date < timezone.now():
       raise ValidationError('You can not create topics for course that has ended.')

    topic = Topic.objects.create(name=name, course=course)

    return topic

Handle Exceptions in APIs

In order to transform the exceptions raised in the services or selectors, to a standard HTTP response, you need to catch the exception and raise something that the rest framework understands.

The best place to do this is in the handle_exception method of the APIView.

There you can map your exception to DRF exception.

Here is an example:

from rest_framework import exceptions as rest_exceptions

from django.core.exceptions import ValidationError


class CourseCreateApi(SomeAuthenticationMixin, APIView):
    expected_exceptions = {
        ValidationError: rest_exceptions.ValidationError
    }

    class InputSerializer(serializers.Serializer):
        ...

    def post(self, request):
        serializer = self.InputSerializer(data=request.data)
        serializer.is_valid(raise_exception=True)

        create_course(**serializer.validated_data)

        return Response(status=status.HTTP_201_CREATED)

    def handle_exception(self, exc):
        if isinstance(exc, tuple(self.expected_exceptions.keys())):
            drf_exception_class = self.expected_exceptions[exc.__class__]
            drf_exception = drf_exception_class(get_error_message(exc))

            return super().handle_exception(drf_exception)

        return super().handle_exception(exc)

Here's the implementation of get_error_message:

def get_first_matching_attr(obj, *attrs, default=None):
    for attr in attrs:
        if hasattr(obj, attr):
            return getattr(obj, attr)

    return default


def get_error_message(exc):
    if hasattr(exc, 'message_dict'):
        return exc.message_dict
    error_msg = get_first_matching_attr(exc, 'message', 'messages')

    if isinstance(error_msg, list):
        error_msg = ', '.join(error_msg)

    if error_msg is None:
        error_msg = str(exc)

    return error_msg

You can move this code to a mixin and use it in every API to prevent code duplication.

We call this ExceptionHandlerMixin. Here's a sample implementation from one of our projects:

from rest_framework import exceptions as rest_exceptions

from django.core.exceptions import ValidationError

from project.common.utils import get_error_message


class ExceptionHandlerMixin:
    """
    Mixin that transforms Django and Python exceptions into rest_framework ones.
    without the mixin, they return 500 status code which is not desired.
    """
    expected_exceptions = {
        ValueError: rest_exceptions.ValidationError,
        ValidationError: rest_exceptions.ValidationError,
        PermissionError: rest_exceptions.PermissionDenied
    }

    def handle_exception(self, exc):
        if isinstance(exc, tuple(self.expected_exceptions.keys())):
            drf_exception_class = self.expected_exceptions[exc.__class__]
            drf_exception = drf_exception_class(get_error_message(exc))

            return super().handle_exception(drf_exception)

        return super().handle_exception(exc)

Having this mixin in mind, our API can be written like that:

class CourseCreateApi(
  SomeAuthenticationMixin,
  ExceptionHandlerMixin,
  APIView
):
    class InputSerializer(serializers.Serializer):
        ...

    def post(self, request):
        serializer = self.InputSerializer(data=request.data)
        serializer.is_valid(raise_exception=True)

        create_course(**serializer.validated_data)

        return Response(status=status.HTTP_201_CREATED)

All of code above can be found in utils.py in this repository.

Inspiration

The way we do Django is inspired by the following things:

• The general idea for separation of concerns
• Boundaries by Gary Bernhardt
• Rails service objects