autowired

A minimalistic dependency injection library for Python.

Installation

pip install autowired

Quick Start

The heart of autowired is the Context class.
You can think of it as a declarative dependency injection container.

Let's look at an example.
We start by defining some components in plain Python:

class ComponentA:
    def hello(self, name: str):
        print(f"Hello, {name}!")


class ComponentB:
    def goodbye(self, name: str):
        print(f"Goodbye, {name}!")


class MainComponent:
    def __init__(self, component_a: ComponentA, component_b: ComponentB):
        self.component_a = component_a
        self.component_b = component_b

    def run(self):
        self.component_a.hello("World")
        self.component_b.goodbye("World")

Next, we'll define the Context class.
In our application code, we only need to interact with the MainComponent, hence it's the only component we explicitly define.

from autowired import Context, autowired


class ApplicationContext(Context):
    main_component: MainComponent = autowired()

Finally, we can utilize it in our application code:

ctx = ApplicationContext()
ctx.main_component.run()

In this example, autowired was able to resolve all dependencies automatically. However, in most real-world applications, you will need more control over the instantiation process. The following sections will explain all the necessary concepts and advanced features in more detail.

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Core Principles

Dependency Injection

Reusability, maintainability, and testability are important aspects of code quality. One technique commonly used for achieving this is Dependency Injection (DI).

In essence, DI is about decoupling the creation of objects from their usage. It encourages a system where dependencies are not built internally, but provided (or 'injected') externally. This approach offers the flexibility to replace dependencies without altering the classes using them.

While some might associate DI with complex frameworks, it's primarily a simple but effective design pattern.

A simple example:

1. Without DI:

class TextWriter:
    def write(self, text: str):
        print(text)


class Poet:
    def __init__(self):
        self.writer = TextWriter()

    def write_poem(self):
        self.writer.write("Roses are red, violets are blue...")

Here, the Poet class is tightly coupled to the TextWriter class. If we wanted to use a different writer, we would have to change the Poet class.

2. With DI:

class TextWriter:
    def write(self, text: str):
        print(text)


class Poet:
    def __init__(self, writer: TextWriter):
        self.writer = writer

    def write_poem(self):
        self.writer.write("Roses are red, violets are blue...")

Here, the Poet class is decoupled from the TextWriter class. It can now interact with any class that implements the TextWriter interface without necessitating changes to the Poet class itself. This elevates the flexibility and reusability of the Poet class. Moreover, it simplifies testing, as the Poet can now be easily tested in isolation from the TextWriter.

Since Dependency Injection relieves the class from creating its own dependencies, these now need to be provided from the outside. This naturally leads to the question: Who takes up this responsibility?

In a simple application, this could be the main function. It could be responsible for reading the configuration and instantiating all the necessary components with the correct dependencies. This is sufficient for small applications, but it quickly becomes unwieldy as the application grows. This is especially true if you have multiple entry points and need to reuse the same instantiation logic in different places such as in a CLI, a web app, or a test suite.

To resolve this, one clean approach is to create a central class that takes over this responsibility and allows access to all the necessary components. These components could be presented as properties of this class, each tied to the others during instantiation.

Typically, it's preferable for multiple components to share the same instance (often called a singleton) of a specific dependency. In such cases, Python’s built-in cached_property decorator is an ideal solution. It functions by saving the result of a property's initial call and then returns this cached value for any subsequent calls.
This effectively provides all that's needed for a simple but elegant and reusable form of Dependency Injection in Python.

Let's look at a more concrete example:

from dataclasses import dataclass


# define some components

class MessageService:
    def send_message(self, user: str, message: str):
        print(f"Sending message '{message}' to user '{user}'")


class UserService:
    def get_user(self, user_id: int):
        return f"User{user_id}"


@dataclass
class NotificationService:
    message_service: MessageService
    user_service: UserService
    all_caps: bool = False

    def send_notification(self, user_id: int, message: str):
        user = self.user_service.get_user(user_id)

        if self.all_caps:
            message = message.upper()

        self.message_service.send_message(user, message)


@dataclass
class NotificationController:
    notification_service: NotificationService

    def notify(self, user_id: int, message: str):
        print(f"Sending notification to user {user_id}")
        self.notification_service.send_notification(user_id, message)

And now we create a central class that ties everything together.

from functools import cached_property


class ApplicationContext:

    @cached_property
    def message_service(self) -> MessageService:
        return MessageService()

    @cached_property
    def user_service(self) -> UserService:
        return UserService()

    @cached_property
    def notification_service(self) -> NotificationService:
        return NotificationService(
            message_service=self.message_service,
            user_service=self.user_service
        )

    @cached_property
    def notification_controller(self) -> NotificationController:
        return NotificationController(
            notification_service=self.notification_service
        )


ctx = ApplicationContext()
ctx.notification_controller.notify(1, "Hello, User!")

In this setup, the ApplicationContext is responsible for managing the dependencies between components. Using the cached_property decorator ensures that each component is instantiated only once, even if it's accessed multiple times.

This approach is already sufficient for many simple applications. However, as the application becomes larger and more complex, the context class can quickly become bloated. You'll have more components, increasing interdependencies, and you'll need to carefully manage the differing life cycles or scopes of each component (e.g. request scoped components, session-scoped ones, etc.). As the complexity grows, so does the amount of boilerplate code needed, making it harder to maintain and increasing the risk for errors. Autowired aims to streamline this process, while building on the same simple principles.

Using autowired

Here's how the previous ApplicationContext could be rewritten using autowired:

from autowired import Context, autowired


class ApplicationContext(Context):
    notification_controller: NotificationController = autowired()

We have simplified the context class to a single line of code. As the NotificationController was the only component that needed to be exposed as a public property, it is the only one we explicitly define. Autowired now handles the instantiation of all components and their dependencies for us. Components can be either dataclasses or traditional classes, provided they are appropriately annotated with type hints for autowired to automatically resolve their dependencies.

Configuration

Autowired provides several ways to configure the instantiation of components within a context. Some of them are more convenient, while others offer more flexibility.

Leveraging cached_property and property methods

Using cached_property and property methods is the most flexible way to configure the instantiation of components, as it gives you full control over the process. As mentioned before, autowired builds on the idea of using cached_property to implement the singleton pattern. That's why cached_property is a first-class citizen in autowired. When autowired resolves dependencies, it does not only respect other autowired fields but also cached_property as well as property methods.

Here is an example of how to make use of this to configure the NotificationService from the previous example:

# We define a dataclass to represent our application settings
@dataclass
class ApplicationSettings:
    all_caps_notifications: bool = False


class ApplicationContext(Context):
    notification_controller: NotificationController = autowired()

    # we add a constructor to the context class to allow passing in the settings
    def __init__(self, settings: ApplicationSettings = ApplicationSettings()):
        self.settings = settings

    @cached_property
    def _notification_service(self) -> NotificationService:
        return self.autowire(
            NotificationService,
            all_caps=self.settings.all_caps_notifications
        )

Now, we can use the context class as before, with the added benefit of being able to configure the notification service via the ApplicationSettings.

settings = ApplicationSettings(all_caps_notifications=True)
ctx = ApplicationContext(settings=settings)
ctx.notification_controller.notify(1, "Hello, User!")

assert ctx.notification_controller.notification_service.all_caps == True

The autowire method behaves very similarly to the autowired field, but it is meant to be used to directly instantiate components, rather than to define them as fields. Explicit dependencies can be passed as kwargs, as shown in the example above, while the remaining ones will be resolved automatically as before.

Configuring Autowired Fields with Context Attributes

Using cached_property and property allows us to define our own factory functions for components. However, for simple use cases, this is unnecessarily verbose. To configure your autowired fields with attributes of the context-instance, you can also directly reference these attributes in the field definition.

Here is how you could rewrite the previous example:

class ApplicationContext(Context):
    settings: ApplicationSettings = provided()
    notification_controller: NotificationController = autowired()
    _notification_service: NotificationService = autowired(all_caps=settings.all_caps_notifications)

    def __init__(self, settings: ApplicationSettings = ApplicationSettings()):
        self.settings = settings

To make the settings field available in the autowired field definition, we need to define it explicitly. Note that we use provided() instead of autowired() because the field is manually set in the constructor.

Which of the two approaches you prefer is a matter of taste or the complexity of evaluating the settings. For simple settings, the second approach should be preferred. For more complex rules, the cached_property approach might be more suitable. Both approaches can be mixed freely.

Advanced Configuration with Kwargs Factory Function

For more complex configuration scenarios, you can use a kwargs factory function with autowired fields. This approach provides a balance between simplicity and flexibility, allowing you to define custom logic for setting up your autowired fields directly in the field definition.

The factory function receives the context instance as its only argument during the component's instantiation. This allows you to access any attribute of the context and use it in your configuration logic. It should return a dictionary of kwargs that will be passed to the component's constructor. As before, the remaining dependencies will be resolved automatically.

Here's how you can apply it:

class ApplicationContext(Context):
    notification_controller: NotificationController = autowired()
    _notification_service: NotificationService = autowired(
        lambda self: dict(all_caps=self.settings.all_caps_notifications)
    )

    def __init__(self, settings: ApplicationSettings = ApplicationSettings()):
        self.settings = settings

As always, you can freely mix and match the approaches within a single context class.

Recap — The Building Blocks

We already covered the most important building blocks of autowired.

• Context serves as the base class for all classes that manage dependencies between components.
• autowired() defines autowired fields.
• @cached_property and @property offer more control over the instantiation process.
• self.autowire() is a helper method for implementing @cached_property and @property methods on context classes.

Advanced Features

Component Lifetime

By default, components function as singletons, meaning the same instance is returned each time they're accessed or injected from a context. However, situations may arise where a different lifetime for a component is required. Autowired offers three specific lifetimes within a context: singleton, transient, and thread-local. These can be applied to both autowired fields and properties, as shown in the table below:

Lifetime	Description	Autowired Syntax	Decorator
Singleton	Single shared instance across the context	autowired()	@cached_property
Transient	A new instance is created whenever accessed or injected	autowired(transient=True)	@property
Thread	Unique instance per thread	autowired(thread_local=True)	@thread_local_cached_property

While component lifetimes dictate the policy for instantiation of components within a particular context, determining whether new instances are created or existing ones are reused, another essential dimension in component lifetime management exists: the lifetime of the context itself. The next sections will describe that in more detail.

Scopes and Derived Contexts

In many applications, components can be bound to a specific scope. A common example is a web application, where some components are request-scoped, while others are session-scoped or application-scoped. Often, these scopes follow a hierarchy; for example, a request scope is part of a session scope, which is part of the application scope.

While it's certainly possible to manage all these components within a single context, it can sometimes be beneficial to break them up into multiple contexts. Each context can then handle its own component instances, while drawing from the parent context if necessary.

The next example demonstrates how this hierarchical structure can be implemented using autowired.

from autowired import Context, autowired, provided
import json
from dataclasses import dataclass


# application scope

class DatabaseService:

    def __init__(self, connection_string: str):
        self.connection_string = connection_string

    def get_api_keys(self):
        print(f"Fetching API keys from the database...")
        return ["123", "456", ""]

    def get_user_data(self, user_id: str):
        print(f"Fetching data for user {user_id} from the database...")
        return {"name": "John Doe", "email": "[email protected]"}


@dataclass
class ApplicationSettings:
    db_connection_string: str = "db://localhost"


class ApplicationContext(Context):
    settings: ApplicationSettings = provided()
    database_service: DatabaseService = autowired(connection_string=settings.db_connection_string)

    def __init__(self, settings: ApplicationSettings):
        self.settings = settings


# request scope

@dataclass
class HttpRequest:
    headers: dict[str, str]
    parameters: dict[str, str]


class HttpRequestHandler:

    def __init__(self, database_service: DatabaseService, http_request: HttpRequest):
        self.database_service = database_service
        self.http_request = http_request

    def handle_request(self) -> str:
        api_key = self.http_request.headers.get("Authorization") or ""
        if api_key in self.database_service.get_api_keys():
            print("User is authorised")
            user_id = self.http_request.parameters.get("user_id")
            user_data = self.database_service.get_user_data(user_id)
            return json.dumps(user_data)
        else:
            raise Exception("Not authorised")


class RequestContext(Context):
    http_request: HttpRequest = provided()
    http_request_handler: HttpRequestHandler = autowired()

    def __init__(self, parent_context: Context, http_request: HttpRequest):
        self.derive_from(parent_context)
        self.http_request = http_request


# example usage

settings = ApplicationSettings(db_connection_string="db://localhost")
app_context = ApplicationContext(settings)

# Create a dummy HTTP request
http_request = HttpRequest(headers={"Authorization": "123"}, parameters={"user_id": "1"})

# Create a request context for the dummy request
request_context = RequestContext(app_context, http_request)

# Use the HttpRequestHandler to handle the request
response = request_context.http_request_handler.handle_request()

print(response)

Eager and Lazy Instantiation

By default, autowired() fields behave like cached_propertys and are instantiated lazily, i.e., the first time they are accessed. If this is not the desired behavior, you can use the eager parameter to force eager instantiation of the component.

class ApplicationContext(Context):
    notification_controller: NotificationController = autowired(eager=True)

The Container

Most of the time, using the Context class is sufficient for managing dependencies between components. However, since it requires knowing upfront which components will be needed, it might not be suitable for all use cases. Therefore, if you need more flexibility, you can use the Container class instead. You can instantiate a container yourself or access a context's container via the container property.

from autowired import Container


class MessageService:
    def send_message(self, user: str, message: str):
        print(f"Sending message '{message}' to user '{user}'")


class UserService:
    def get_user(self, user_id: int):
        return f"User{user_id}"


class NotificationService:
    def __init__(self, message_service: MessageService, user_service: UserService):
        self.message_service = message_service
        self.user_service = user_service

    def send_notification(self, user_id: int, message: str):
        user = self.user_service.get_user(user_id)
        self.message_service.send_message(user, message)


container = Container()
notification_service = container.resolve(NotificationService)

assert isinstance(notification_service, NotificationService)
assert notification_service is container.resolve(NotificationService)
assert notification_service.message_service is container.resolve(MessageService)

For more information on how to use the Container class, refer to its code documentation.

Provider

A container can contain a list of providers (instances of the Provider class). A provider is what actually creates the instances of a component. Most of the time, especially when using the Context class, you don't need to worry about providers, as they are created automatically. The Provider class defines a simple interface that the Container class uses to resolve dependencies.

class Provider(Generic[T]):

    def satisfies(self, dependency: Dependency) -> bool:
        # Checks whether the provider can provide an instances that satisfies the given dependency specification.
        ...

    def get_instance(self, dependency: Dependency, container: Container) -> T:
        # Returns an instance that satisfies the given dependency specification.
        ...

    def get_name(self) -> str:
        # Each provider has a name. The container utilises it to resolve ambiguous dependencies.
        ...

Most providers are singleton component providers, i.e., they always return the same instance when get_instance() is called. In the above container usage example, when we resolved the NotificationService for the first time, a singleton provider was created automatically and added to the container. However, you can also add providers manually.
In most cases you use the from_supplier or from_instance factory methods to create a provider, but you can also implement your own Provider subclass. In the following example, we use the from_supplier factory method to create a transient provider for a custom MessageService class.

from autowired import Container, Provider

container = Container()


class AllCapsMessageService(MessageService):
    def send_message(self, user: str, message: str):
        super().send_message(user, message.upper())


def create_message_service() -> MessageService:
    return AllCapsMessageService()


# Using `from_supplier` calls the given supplier function each time 
# Note that the return type annotation on the supplier function is mandatory
# unless you specify the type argument explicitly
container.add(Provider.from_supplier(create_message_service))

assert isinstance(container.resolve(MessageService), AllCapsMessageService)
assert container.resolve(MessageService) is not container.resolve(MessageService)

List Injection

Sometimes, you might want to inject a list of all components that implement a specific interface. This is especially useful when you want to implement a plugin system.

from autowired import Context, autowired
from abc import ABC, abstractmethod


class Plugin(ABC):
    @abstractmethod
    def run(self):
        ...


class PluginA(Plugin):
    def run(self):
        print("Plugin A")


class PluginB(Plugin):
    def run(self):
        print("Plugin B")


class PluginManager:
    def __init__(self, plugins: list[Plugin]):
        self.plugins = plugins

    def run_all(self):
        for plugin in self.plugins:
            plugin.run()


class ApplicationContext(Context):
    plugin_manager: PluginManager = autowired()


# usage

ctx = ApplicationContext()

ctx.container.add(PluginA())
ctx.container.add(PluginB())

ctx.plugin_manager.run_all()

Component Scan

In many applications, you might want to automatically discover all components in a specific package. Like list injection, this can be useful for implementing a plugin system. Another common use case is to automatically discover all controllers in a web application to easily set up routing. You can use the @component decorator to mark a class as a component. When you call component_scan() on a container, it will automatically discover all components in the given package and add them to the container.

# my_module/controllers/controller.py

from abc import ABC, abstractmethod


class Controller(ABC):
    @abstractmethod
    def run(self):
        ...


# my_module/controllers/controller1.py
from autowired import component
from .controller import Controller


@component
class Controller1(Controller):
    def run(self):
        print("Starting Controller 1")


# my_module/controllers/controller2.py
from autowired import component
from .controller import Controller


@component
class Controller2(Controller):
    def run(self):
        print("Starting Controller 2")


# my_module/main.py

from autowired import Context, autowired
import my_module.controllers


class ControllerManager:
    def __init__(self, controllers: list[Controller]):
        self.controllers = controllers

    def start(self):
        for controller in self.controllers:
            controller.run()

class ApplicationContext(Context):
    controller_manager: ControllerManager = autowired()
    
    def __init__(self):
        # register all components from the my_module.controllers package
        self.container.component_scan(my_module.controllers)


# usage

ctx = ApplicationContext()
ctx.controller_manager.start()