#6 Dependency Injection

Module 6 — Dependency Injection

FastAPI has a powerful and elegant Dependency Injection (DI) system. It allows you to extract shared logic (e.g., authentication, DB connections, validation).

In this module, you will learn:

• Simple function dependencies
• Class-based dependencies
• Dependencies with parameters
• Using dependencies in routers

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1. Basic Dependency

A dependency is a function that FastAPI calls before the endpoint.

from fastapi import FastAPI, Depends

app = FastAPI()

def get_token():
    return "sample-token"

@app.get("/secure")
def secure_route(token: str = Depends(get_token)):
    return {"token": token}

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2. Dependency with Logic

You can place reusable validation or computation here.

from fastapi import FastAPI, Depends, HTTPException

app = FastAPI()

def validate_api_key(api_key: str = "secret123"):
    if api_key != "secret123":
        raise HTTPException(status_code=401, detail="Invalid API Key")
    return api_key

@app.get("/protected")
def protected_route(key: str = Depends(validate_api_key)):
    return {"status": "Access granted"}

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3. Dependency with Parameters

from fastapi import FastAPI, Depends

app = FastAPI()

def pagination(skip: int = 0, limit: int = 10):
    return {"skip": skip, "limit": limit}

@app.get("/items")
def get_items(settings: dict = Depends(pagination)):
    return settings

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4. Class-Based Dependencies

You can use classes as auto-initialized dependencies.

from fastapi import FastAPI, Depends

app = FastAPI()

class DbSession:
    def __init__(self):
        self.conn = "Connected to fake DB"

def get_db():
    return DbSession()

@app.get("/db")
def db_route(db: DbSession = Depends(get_db)):
    return {"db_status": db.conn}

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5. Dependencies in Routers (Advanced)

from fastapi import FastAPI, APIRouter, Depends

app = FastAPI()

def auth_user():
    return "Authorized User"

router = APIRouter(
    prefix="/users",
    dependencies=[Depends(auth_user)]
)

@router.get("/")
def get_users():
    return ["User1", "User2"]

app.include_router(router)

Every endpoint inside the router now automatically uses auth_user().

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6. Complete Dependency Demo (Copy & Run)

from fastapi import FastAPI, APIRouter, Depends, HTTPException

app = FastAPI()

# Simple dependency
def get_token():
    return "abc123"

# Parameterized dependency
def pagination(skip: int = 0, limit: int = 10):
    return {"skip": skip, "limit": limit}

# Auth dependency
def check_auth(api_key: str = "secret123"):
    if api_key != "secret123":
        raise HTTPException(status_code=401, detail="Unauthorized")
    return api_key

# Router with dependency
router = APIRouter(prefix="/users", dependencies=[Depends(check_auth)])

@router.get("/")
def list_users():
    return ["Alice", "Bob", "Charlie"]

app.include_router(router)

@app.get("/secure")
def secure_route(token: str = Depends(get_token)):
    return {"token": token}

@app.get("/items")
def items(pagination_settings: dict = Depends(pagination)):
    return pagination_settings

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