What is FastAPI dependency injection?

Question

Accepted Answer

**FastAPI DI** manages **shared resources** (DB sessions, auth) via `Depends()`.

```python
from fastapi import FastAPI, Depends, HTTPException
from sqlalchemy.orm import Session
from typing import Annotated

app = FastAPI()

# Database dependency
async def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

DbSession = Annotated[Session, Depends(get_db)]

# Auth dependency
async def get_current_user(
    token: str,
    db: DbSession
) -> User:
    user = verify_token(db, token)
    if not user:
        raise HTTPException(status_code=401)
    return user

CurrentUser = Annotated[User, Depends(get_current_user)]

# Using dependencies
@app.get("/users/me")
async def read_users_me(current_user: CurrentUser):
    return current_user

@app.get("/items/")
async def read_items(db: DbSession, user: CurrentUser):
    return db.query(Item).filter(Item.owner == user.id).all()
```

**Benefits:**
- Automatic request parameter injection.
- Reusable logic across endpoints.
- Easy testing (mock dependencies).
- Automatic OpenAPI documentation.

Python: What is FastAPI dependency injection?