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

FastAPI provides OAuth2PasswordBearer and OAuth2PasswordRequestForm helpers for the standard username/password token flow. The pattern: client POSTs credentials → server returns a JWT → client sends JWT in Authorization: Bearer <token> header on subsequent requests.

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}

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