Overview

It's a bit old article, but InfoQ's Domain Driven Design and Development Practices says:

The design dependency that the domain class depends on the Data Access Object (DAO) class and the service class depends on the domain class must "DI" for implementation by DDD. "I'm making it.

For Python, Inject is useful for achieving DI (Dependency Injection).

Usage example

If you write a sample with roughly 100 lines, it will look like this.

# -*- coding: utf-8 -*-
import uuid
from abc import ABCMeta, abstractmethod

import inject


def config(binder):
    binder.bind(UserRepository, UserMemoryRepository())


class User(object):

    def __init__(self, identity, name):
        self.identity = identity
        self.name = name


class UserRepository(object):
    u""" Base class of User Repository"""
    __metaclass__ = ABCMeta

    @abstractmethod
    def store(self, user):
        raise NotImplementedError

    @abstractmethod
    def find_by_identity(self, identity):
        raise NotImplementedError


class UserMemoryRepository(UserRepository):
    u""" User Repository on memory"""

    def __init__(self):
        self._users = {}

    def store(self, user):
        if not isinstance(user, User):
            raise TypeError
        self._users[user.identity] = user

    def find_by_identity(self, identity):
        return self._users[identity]


class UserRedisRepository(UserRepository):
    u""" User Repository on Redis """

    def store(self, user):
        # TODO: write code here
        pass

    def find_by_identity(self, identity):
        # TODO: write code here
        pass


class UserService(object):
    u""" User Service on Application Layer"""
    repo = inject.attr(UserRepository)

    def create_user(self, name):
        user = User(uuid.uuid4(), name)
        self.repo.store(user)
        return user

    def find_by_identity(self, identity):
        return self.repo.find_by_identity(identity)


if __name__ == "__main__":
    # Call once on application start
    inject.configure(config)

    user_service = UserService()
    created_user = user_service.create_user('foo')
    stored_user = user_service.find_by_identity(created_user.identity)

    assert created_user == stored_user

To achieve the same thing without using ʻinject, you need to pass repo as an argument to ʻUserService, which makes the constructor of ʻUserService bloated as the number of dependencies increases. , ʻInject can be used to write clearly. You can also use the @ inject.params decorator to inject dependencies into the default arguments of the constructor.

It's a small library with only 300 lines, but it's very useful.

Inject is recommended for DDD in Python

Overview

Usage example