yggsdrasyl

Simplistic, but totally explicit dependency injection tool. Main goal is to provide simple, but powerful API for registering dependencies by type and their extensible resolution mechanism in functions.

Why yggdrasyl? This is misspelling of yggdrasil - the ancient tree of life in Nordic / Scandinavian mythologies, that connects all the worlds, like DI Framework connects dependencies.

Usage

Main entry point to dependency management is class Dependencies. You can create your own instance, or use predefined global one - deps.

Dependency Registration

To register a dependency invoke method Dependencies.register:

from yggdrasyl import deps


deps.register(UserServiceConfig, lambda d: UserServiceConfig())
deps.register(UserService, lambda d: UserService(config=d.resolve(UserServiceConfig)))

Dependencies.register accepts accepts 2 required arguments:

1. dependency type - identifier for dependency resolution
2. factory function - function that accepts Dependencies object and returns dependency instance

Also there are multiple optional keyword-only arguments:

1. cached - if True than dependency object is reused across all resolutions, True by default
2. managed - if True than dependency object is managed by context manager and can be initialized with all the other dependencies
3. override - if True than existing dependency with same type is overridden, False by default

For simplification of registering objects from instances one can use from_instance utility function. If one has factory function (for example class constructor) use from_factory.

from yggdrasyl import deps, from_instance, from_factory


deps.register(int, from_instance(1))
deps.register(IService, from_factory(Service))

If dependency type is already registered, then TypeAlreadyRegisteredError is raised.

Dependency Resolution

To resolve dependency invoke Dependencies.resolve method with single desired type argument:

from yggdrasyl import deps

user_service = deps.resolve(UserService)

If type is not registered, then TypeNotRegisteredError is raised.

Context Manager Dependencies

Some dependencies require being initialized on application startup. In order not to forget one and rule management from single place there is a managed parameter, that indicates that this dependency should be resolved on startup and is sync or async context manager, that should be initialized. By default it takes False meaning, that one must explicitly tell, that dependency is managed.

After marking only dependencies as managed just open Dependencies.initialize context and all the dependencies will be initialized at once. Basically this method should be used as lifespan for ASGI applications (for example).

from contextlib import asynccontextmanager

from fastapi import FastAPI
from yggdrasyl import deps


@asynccontextmanager
async def _lifespan(app: FastAPI):
    async with deps.initialize():
        yield

Dependency Wiring

In order not to call Dependencies.resolve manually for each dependency, there is a way to tell that some callable requires some dependencies via Dependencies.wire decorator:

from yggdrasyl import deps, Injected


@deps.wire
async def _handle_register_user(
    action: RegisterUser,
    user_service: UserService = Injected[UserService]
) -> UserID:
    ...

To mark argument to be injected from DI container one should use Injected object with wanted type in [].