magic-di

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Documentation: https://woltapp.github.io/magic-di

Source Code: https://github.com/woltapp/magic-di

PyPI: https://pypi.org/project/magic-di/

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Dependency Injector with minimal boilerplate code, built-in support for FastAPI and Celery, and seamless integration to basically anything.

Contents

• Install
• Getting Started
• Clients Configuration
  • Zero config clients
  • Clients with Config
• Using interfaces instead of implementations
• Integration with Celery
  • Function based tasks
  • Class based tasks
• Custom integrations
  • Manual injection
• Forced injections
• Healthcheck
• Testing
  • Default simple mock
  • Custom mocks
• Alternatives
• Development

Install

pip install magic-di

With FastAPI integration:

pip install 'magic-di[fastapi]'

With Celery integration:

pip install 'magic-di[celery]'

Getting Started

from fastapi import FastAPI

from magic_di import Connectable
from magic_di.fastapi import inject_app, Provide

app = inject_app(FastAPI())


class Database:
    connected: bool = False

    def __connect__(self):
        self.connected = True

    def __disconnect__(self):
        self.connected = False


class Service(Connectable):
    def __init__(self, db: Database):
        self.db = db

    def is_connected(self):
        return self.db.connected


@app.get(path="/hello-world")
def hello_world(service: Provide[Service]) -> dict:
    return {
        "is_connected": service.is_connected()
    }

That's all!

This simple code will recursively inject all dependencies and connect them using the __connect__ and __disconnect__ magic methods.

But what happened there?

1. We created a new FastAPI app and injected it. The inject_app function makes the injector connect all clients on app startup and disconnect them on shutdown. That’s how you can open and close all connections (e.g., session to DB).
2. We defined new classes with __connect__ and __disconnect__ magic methods. That’s how the injector finds classes that need to be injected. The injector uses duck typing to check if some class has these methods. It means you don’t need to inherit from ClientProtocol (but you can to reduce the number of code lines).
3. Wrapped the Service type hint into Provide so that FastAPI can use our DI. Please note: you need to use Provide only in FastAPI endpoints, which makes your codebase independent from FastAPI and this Dependency Injector.
4. PROFIT!

As you can see, in this example, you don’t need to write special constructors to store your dependencies in global variables. All you need to do to complete the startup logic is to write it in the __connect__ method.

Clients Configuration

This dependency injector promotes the idea of ‘zero-config clients’, but you can still use configurations if you prefer

Zero config clients

Simply fetch everything needed from the environment. There is no need for an additional configuration file

from dataclasses import dataclass, field

from pydantic import Field
from pydantic_settings import BaseSettings
from redis.asyncio import Redis as RedisClient, from_url


class RedisConfig(BaseSettings):
    url: str = Field(validation_alias='REDIS_URL')
    decode_responses: bool = Field(validation_alias='REDIS_DECODE_RESPONSES')


@dataclass
class Redis:
    config: RedisConfig = field(default_factory=RedisConfig)
    client: RedisClient = field(init=False)

    async def __connect__(self):
        self.client = await from_url(self.config.url, decode_responses=self.config.decode_responses)
        await self.client.ping()

    async def __disconnect__(self):
        await self.client.close()

    @property
    def db(self) -> RedisClient:
        return self.client


Redis()  # works even without passing arguments in the constructor.

As an alternative, you can inject configs instead of using default factories.

from dataclasses import dataclass, field

from pydantic import Field
from pydantic_settings import BaseSettings
from redis.asyncio import Redis as RedisClient, from_url

from magic_di import Connectable, DependencyInjector


class RedisConfig(Connectable, BaseSettings):
    url: str = Field(validation_alias='REDIS_URL')
    decode_responses: bool = Field(validation_alias='REDIS_DECODE_RESPONSES')


@dataclass
class Redis:
    config: RedisConfig
    client: RedisClient = field(init=False)

    async def __connect__(self):
        self.client = await from_url(self.config.url, decode_responses=self.config.decode_responses)
        await self.client.ping()

    async def __disconnect__(self):
        await self.client.close()

    @property
    def db(self) -> RedisClient:
        return self.client


injector = DependencyInjector()
redis = injector.inject(Redis)()  # works even without passing arguments in the constructor.

async with injector:
    await redis.db.ping()

Using interfaces instead of implementations

Sometimes, you may not want to stick to a certain interface implementation everywhere. Therefore, you can use interfaces (protocols, abstract classes) with Dependency Injection (DI). With DI, you can effortlessly bind an implementation to an interface and subsequently update it if necessary.

from typing import Protocol

from fastapi import FastAPI

from magic_di import Connectable, DependencyInjector
from magic_di.fastapi import inject_app, Provide


class MyInterface(Protocol):
    def do_something(self) -> bool:
        ...


class MyInterfaceImplementation(Connectable):
    def do_something(self) -> bool:
        return True


app = inject_app(FastAPI())

injector = DependencyInjector()
injector.bind({MyInterface: MyInterfaceImplementation})


@app.get(path="/hello-world")
def hello_world(service: Provide[MyInterface]) -> dict:
    return {
        "result": service.do_something(),
    }

Using injector.bind, you can bind implementations that will be injected everywhere the bound interface is used.

Integration with Celery

Function based celery tasks

from celery import Celery

from magic_di.celery import get_celery_loader, InjectableCeleryTask, PROVIDE

app = Celery(
    loader=get_celery_loader(),
    task_cls=InjectableCeleryTask,
)


@app.task
async def calculate(x: int, y: int, calculator: Calculator = PROVIDE):
    await calculator.calculate(x, y)

Class based celery tasks

from dataclasses import dataclass

from celery import Celery

from magic_di.celery import get_celery_loader, InjectableCeleryTask, BaseCeleryConnectableDeps, PROVIDE

app = Celery(
    loader=get_celery_loader(),
    task_cls=InjectableCeleryTask,
)


@dataclass
class CalculatorTaskDeps(BaseCeleryConnectableDeps):
    calculator: Calculator


class CalculatorTask(InjectableCeleryTask):
    deps: CalculatorTaskDeps

    async def run(self, x: int, y: int, smart_processor: SmartProcessor = PROVIDE):
        return smart_processor.process(
            await self.deps.calculator.calculate(x, y)
        )


app.register_task(CalculatorTask)

Limitations

You could notice that in these examples tasks are using Python async/await. InjectableCeleryTask provides support for writing async code. However, it still executes code synchronously. Due to this, getting results from async tasks is not possible in the following cases:

• When the task_always_eager config flag is enabled and task creation occurs inside the running event loop (e.g., inside an async FastAPI endpoint)
• When calling the .apply() method inside running event loop (e.g., inside an async FastAPI endpoint)

Custom integrations

For custom integration you can either use helper function inject_and_run or by using DependencyInjector manually

from magic_di.utils import inject_and_run


async def main(worker: Worker):
    await worker.run()

if __name__ == '__main__':
    inject_and_run(main)

Manual injection

import asyncio

from magic_di import DependencyInjector


async def run_worker(worker: Worker):
    await worker.run()


async def main():
    injector = DependencyInjector()

    injected_fn = injector.inject(run_worker)

    async with injector:
        await injected_fn()

if __name__ == '__main__':
    asyncio.run(main())

Forced injections

You can force injector to inject non-connectable dependencies with type hint annotation Injectable

from typing import Annotated

from magic_di import Injectable, Connectable


class Service(Connectable):
    dependency: Annotated[NonConnectableDependency, Injectable]

Healthcheck

You can implement Pingable protocol to define healthchecks for your clients. The DependenciesHealthcheck will call the __ping__ method on all injected clients that implement this protocol.

from magic_di.healthcheck import DependenciesHealthcheck


class Service(Connectable):
    def __init__(self, db: Database):
        self.db = db

    def is_connected(self):
        return self.db.connected

    async def __ping__(self) -> None:
        if not self.is_connected():
            raise Exception("Service is not connected")


@app.get(path="/hello-world")
def hello_world(service: Provide[Service]) -> dict:
    return {
        "is_connected": service.is_connected()
    }


@app.get(path="/healthcheck")
async def healthcheck_handler(healthcheck: Provide[DependenciesHealthcheck]) -> dict:
    await healthcheck.ping_dependencies()
    return {"alive": True}

Testing

If you need to mock a dependency in tests, you can easily do so by using the injector.override context manager and still use this dependency injector.

To mock clients, you can use InjectableMock from the testing module.

Default simple mock

import pytest
from fastapi.testclient import TestClient
from my_app import app

from magic_di import DependencyInjector
from magic_di.testing import InjectableMock


@pytest.fixture()
def injector():
    return DependencyInjector()


@pytest.fixture()
def service_mock() -> Service:
    return InjectableMock()


@pytest.fixture()
def client(injector: DependencyInjector, service_mock: InjectableMock):
    with injector.override({Service: service_mock.mock_cls}):
        with TestClient(app) as client:
            yield client


def test_http_handler(client):
    resp = client.post('/hello-world')

    assert resp.status_code == 200

Custom mocks

As an alternative, you can your use custom mocks.

from magic_di.testing import get_injectable_mock_cls


@pytest.fixture()
def service_mock() -> Service:
    return SomeSmartServiceMock()


@pytest.fixture()
def client(injector: DependencyInjector, service_mock: Service):
    with injector.override({Service: get_injectable_mock_cls(service_mock)}):
        with TestClient(app) as client:
            yield client

Alternatives

FastAPI's built-in dependency injection

FastAPI's built-in DI is great, but it makes the project (and its business logic) dependent on FastAPI, fastapi.Depends specifically.

magic-di decouples DI from other dependencies while still offering seamless integration to FastAPI, for example.

python-dependency-injector

python-dependency-injector is great, but it requires a notable amount of boilerplate code.

The goal of magic-di is to reduce the amount of code as much as possible and get rid of enterprise code with countless configs, containers, and fabrics. The philosophy of magic-di is that clients know how to configure themselves and perform all startup routines.

Development

• Clone this repository
• Requirements:
  • Poetry
  • Python 3.10+
• Create a virtual environment and install the dependencies

poetry install --all-extras

• Activate the virtual environment

poetry shell

Testing

pytest

Documentation

The documentation is automatically generated from the content of the docs directory and from the docstrings of the public signatures of the source code. The documentation is updated and published as a Github Pages page automatically as part each release.

Releasing

Trigger the Draft release workflow (press Run workflow). This will update the changelog & version and create a GitHub release which is in Draft state.

Find the draft release from the GitHub releases and publish it. When a release is published, it'll trigger release workflow which creates PyPI release and deploys updated documentation.

Pre-commit

Pre-commit hooks run all the auto-formatting (ruff format), linters (e.g. ruff and mypy), and other quality checks to make sure the changeset is in good shape before a commit/push happens.

You can install the hooks with (runs for each commit):

pre-commit install

Or if you want them to run only for each push:

pre-commit install -t pre-push

Or if you want e.g. want to run all checks manually for all files:

pre-commit run --all-files

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This project was generated using the wolt-python-package-cookiecutter template.