1. The get_current_user Dependency#

This is the heart of your security. It acts as a gatekeeper for your protected routes.

from jose import JWTError, jwt
from fastapi import Depends, HTTPException, status
from .database import get_db

async def get_current_user(token: str = Depends(oauth2_scheme), db: Session = Depends(get_db)):
    credentials_exception = HTTPException(
        status_code=status.HTTP_401_UNAUTHORIZED,
        detail="Could not validate credentials",
        headers={"WWW-Authenticate": "Bearer"},
    )
    try:
        # 1. Decode the Token
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")
        if username is None:
            raise credentials_exception
    except JWTError:
        raise credentials_exception
        
    # 2. Fetch User from DB
    user = db.query(models.User).filter(models.User.username == username).first()
    if user is None:
        raise credentials_exception
    return user

2. Using the Dependency in Routes#

Now, protecting a route is a one-liner. You don’t have to write any decoding logic in your actual business functions.

@app.get("/users/me")
async def read_users_me(current_user: Annotated[User, Depends(get_current_user)]):
    return current_user

3. Why Dependency Chaining is a Superpower#

FastAPI handles the “tree” of requirements for you. When a request hits /users/me:

1. It sees it needs get_current_user.
2. get_current_user needs oauth2_scheme (the token) and get_db.
3. FastAPI resolves all of them in order and passes the final user object to your function.

🛠️ Implementation Checklist#

• Created the get_current_user function.
• Integrated JWT decoding logic and database lookup.
• Added Annotated for cleaner dependency syntax.
• Verified that /users/me returns the correct user profile when provided a valid token.
• Confirmed that an expired or tampered token results in a 401 Unauthorized.

📚 Resources#

1. Official Docs: FastAPI Security - Get Current User
2. Book: FastAPI: Modern Python Web Development (Chapter 7: Finalizing Authentication).