How do you implement OAuth2 password flow with JWT tokens in FastAPI?

from fastapi import FastAPI, Depends, HTTPException, status
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from typing import Annotated
from datetime import datetime, timedelta
import jwt  # pip install PyJWT

app = FastAPI()
SECRET_KEY = "your-secret-key"  # use a strong random key in production!
ALGORITHM = "HS256"

# Tells FastAPI where clients obtain tokens — used in OpenAPI docs
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")

# 1. Login endpoint — returns a JWT
@app.post("/token")
def login(form: Annotated[OAuth2PasswordRequestForm, Depends()]):
    if form.username != "alice" or form.password != "secret":
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Incorrect username or password",
            headers={"WWW-Authenticate": "Bearer"},
        )
    # Create JWT token
    payload = {
        "sub": form.username,
        "exp": datetime.utcnow() + timedelta(minutes=30),
    }
    token = jwt.encode(payload, SECRET_KEY, algorithm=ALGORITHM)
    return {"access_token": token, "token_type": "bearer"}

# 2. Dependency that decodes and validates the token
def get_current_user(token: Annotated[str, Depends(oauth2_scheme)]):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username = payload.get("sub")
        if not username:
            raise ValueError
    except Exception:
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Could not validate credentials",
            headers={"WWW-Authenticate": "Bearer"},
        )
    return username

# 3. Protected endpoint
@app.get("/me")
def read_me(current_user: Annotated[str, Depends(get_current_user)]):
    return {"username": current_user}