Fast Depends

• Depends is one of the most powerful features in FastAPI.

• It gives us Dependency Injection (DI)

• A clean way to reuse logic, enforce rules, and manage shared resources like authentication, DB sessions, caching, etc.

• 👉 Pydantic = Validates & parses data.

• 👉 Depends = Injects reusable logic/resources.

1. What is Depends?

• Depends is FastAPI’s Dependency Injection (DI) system.
• It tells FastAPI: 👉 “Before executing my endpoint, run this function and give me its result.”

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2. Why Use Depends?

• ✅ Reusability → share logic across endpoints (auth, db sessions).
• ✅ Clean code → keep endpoints small and focused.
• ✅ Testability → mock dependencies easily.
• ✅ Security → centralize auth/permissions.

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3. Basic Example

from fastapi import FastAPI, Depends, HTTPException

app = FastAPI()

# Dependency function
def verify_token(token: str = "test123"):
    if token != "test123":
        raise HTTPException(status_code=403, detail="Invalid token")
    return token

@app.get("/secure-data/")
def secure_data(token: str = Depends(verify_token)):
    return {"msg": "Secure data accessed!", "token": token}

✔️ FastAPI:

1. Calls verify_token before endpoint runs.
2. Injects its result (token) into the endpoint.

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4. Advanced Uses

(a) Database Session

from sqlalchemy.orm import Session
from database import SessionLocal

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

@app.get("/users/")
def get_users(db: Session = Depends(get_db)):
    return db.query(User).all()

(b) Authentication

def get_current_user(token: str = Depends(verify_token)):
    return {"username": "admin"}

(c) Reusable Query Parameters

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

@app.get("/items/")
def list_items(params: dict = Depends(common_params)):
    return {"params": params}

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5. Nested Dependencies

Dependencies can depend on other dependencies:

def get_token_header(token: str = "test123"):
    if token != "test123":
        raise HTTPException(403, "Invalid Token")
    return token

def get_current_user(token: str = Depends(get_token_header)):
    return {"username": "admin"}

@app.get("/profile/")
def profile(user: dict = Depends(get_current_user)):
    return user

Flow: Request → get_token_header → get_current_user → endpoint

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6. Key Benefits in FastAPI

• ✅ Runs dependencies before endpoint
• ✅ Injects results automatically
• ✅ Supports async and sync
• ✅ Plays nicely with Pydantic (validation + DI)
• ✅ Auto-docs include dependencies

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7. How FastAPI Uses Depends Under the Hood

Request cycle with Depends:

Client Request
     ↓
FastAPI Router
     ↓
Evaluate Dependencies (via Depends)
     ↓
Inject Dependency Results into Endpoint
     ↓
Run Endpoint Logic
     ↓
Return Response