dependency-injection-fastapi

FastAPI Dependency Injection

Use FastAPI's Depends() system to inject shared logic into endpoints: database sessions, authentication, authorization, pagination, and configuration. Dependencies are composable, testable, and handle cleanup automatically.

When to Use

• User creates FastAPI endpoints with shared setup/teardown logic
• User implements authentication or authorization
• User manages database connections or sessions
• User asks about dependency injection or testing FastAPI
• User has duplicated logic across multiple endpoints

Core Patterns

Basic Dependencies

from fastapi import Depends, FastAPI, Query

app = FastAPI()

# Simple dependency -- function that returns a value
async def common_parameters(
    skip: int = Query(0, ge=0),
    limit: int = Query(100, ge=1, le=1000),
    sort_by: str = Query("created_at"),
):
    return {"skip": skip, "limit": limit, "sort_by": sort_by}

@app.get("/items")
async def list_items(params: dict = Depends(common_parameters)):
    return await fetch_items(**params)

@app.get("/users")
async def list_users(params: dict = Depends(common_parameters)):
    return await fetch_users(**params)

Database Session Dependencies

Use generator dependencies for automatic session cleanup.

from typing import AsyncGenerator
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine, async_sessionmaker

engine = create_async_engine("postgresql+asyncpg://user:pass@localhost/db")
SessionLocal = async_sessionmaker(engine, expire_on_commit=False)

async def get_db() -> AsyncGenerator[AsyncSession, None]:
    """Yield a database session, ensuring cleanup on exit."""
    async with SessionLocal() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise

@app.get("/users/{user_id}")
async def get_user(user_id: int, db: AsyncSession = Depends(get_db)):
    result = await db.execute(select(User).where(User.id == user_id))
    user = result.scalar_one_or_none()
    if user is None:
        raise HTTPException(status_code=404, detail="User not found")
    return user