Dependency Injection

Code

from fastapi import FastAPI, Depends, Header, HTTPException

app = FastAPI()

def common_params(q: str | None = None, skip: int = 0, limit: int = 10):
    return {"q": q, "skip": skip, "limit": limit}

def verify_token(x_token: str = Header()):
    if x_token != "secret":
        raise HTTPException(status_code=400, detail="bad token")
    return x_token

def get_db():
    db = open_db()
    try:
        yield db
    finally:
        db.close()

@app.get("/items")
def list_items(commons: dict = Depends(common_params), token: str = Depends(verify_token)):
    return commons

@app.get("/users")
def list_users(db = Depends(get_db)):
    return db.query("users")

Explanation

Dependencies declared with Depends are resolved by FastAPI and injected into the handler, which encourages reusable logic for params, auth, and resources. A generator dependency with yield runs setup before and cleanup after the request. Sub-deps and caching make complex wiring concise.

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