dipin

dipin provides a dependency injection container, that primarily aims to work well with FastAPI.

Motivation

FastAPI's dependency container works well, but can be messy to scale. Out of the box, you can end up with many Annotated[X, Depends()]. Some projects I've worked on have a sprawling di.py, and references to Depends in non-FastAPI code.

dipin aims to:

1. Remove the need to annotate types in non-FastAPI code with Depends,
2. Provide a simple API to access dependencies in route-handlers, e.g.:

   from dipin import DI
   
   @app.get("/")
   async def homepage(request: Request, user: DI[AuthenticatedUser]):
       ...
   

   vs

   from .auth import get_authenticated_user
   
   @app.get("/")
   async def homepage(request: Request, user: Annotated[User, Depends(get_authenticated_user)]):
       ...
   

Architecture

TODO

Usage (alpha, buggy, will change)

# di.py

from dipin import DI

# Register a singleton across all requests
from .settings import Settings  # e.g. a pydantic-settings model
DI.register_instance(Settings())

# Register a factory (lazy singleton), called once across all requests
from sqlalchemy.ext.asyncio import create_async_engine
from sqlalchemy.ext.asyncio.engine import AsyncEngine

def create_engine(settings: DI[Settings]) -> AsyncEngine:
    return create_async_engine(str(settings.database_dsn))

DI.register_factory(AsyncEngine, get_db_engine, create_once=True)

# Register a factory called per-request
from sqlmodel.ext.asyncio.session import AsyncSession
from sqlalchemy.ext.asyncio import async_sessionmaker

async def create_session(engine: DI[AsyncEngine]) -> AsyncSession:
    sessionmaker = async_sessionmaker(
        bind=engine, class_=AsyncSession, expire_on_commit=False
    )
    async with sessionmaker() as session:
        yield session

DI.register_factory(AsyncSession, create_session)

# app.py

from fastapi import FastAPI
from sqlmodel.ext.asyncio.session import AsyncSession

from dipin import DI

app = FastAPI()

@app.get("/")
async def list_orders(session: DI[AsyncSession]):
    qry = "SELECT * FROM orders"
    res = await session.exec(qry)
    orders = res.all()

    return {"orders": orders}

Roadmap

• Support default arguments in factories
  • e.g. class A: def __init__(self, foo: str = "bar") should be instantiable.
• Named dependencies
  • e.g. DI["PrivateOpenAIClient"]
  • Out of the box, it seems to lookup the class name (as a deferred annotation), but typing.Literal supports this.
• Full typing support
  • Currently DI[Class] does not resolve to an instance of Class.
• Support many types of dependencies
  • Including named (but we won't be able to use DI[...]), and some of those described in python-dependency-injector.
  • Providing our own fastapi.Depends is likely needed.