user: How to setup authorization and authentication fastapi with explaination

assistant: **FastAPI** is a modern web framework that supports security, authentication and authorization. It has built-in support for OAuth2, which is a standard protocol for handling user authentication and authorization using tokens. FastAPI also provides several tools to help you implement security features easily and rapidly.

One of the ways to implement authentication and authorization in FastAPI is to use the password flow with a bearer token. This means that the user sends their username and password to the API, and the API responds with a token that can be used to access protected resources or perform actions.

To use this method, you need to do the following steps:

- Create an instance of OAuth2PasswordBearer with the URL where the user will send their credentials (e.g. tokenUrl="token").
- Create a dependency function that uses the OAuth2PasswordBearer instance to get the token from the request header and verify it.
- Use the dependency function as a parameter in your protected path operations to authorize the user.

Let me know if you need anymore help?

user: Could you provide a small code example in python implementing this?

assistant: Certainly! Here's a simplified example of how you could implement authorization and authentication using FastAPI in Python:

```python
from fastapi import FastAPI, Depends, HTTPException
from fastapi.security import OAuth2PasswordBearer
from passlib.context import CryptContext
from pydantic import BaseModel
import jwt
import datetime

app = FastAPI()

# Configuration
SECRET_KEY = "mysecretkey"  # Replace with your own secret key
ALGORITHM = "HS256"  # JWT algorithm
ACCESS_TOKEN_EXPIRE_MINUTES = 30  # Access token expiration time in minutes

# Password hashing
pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")

# OAuth2PasswordBearer for token retrieval from request headers
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

# User model
class User(BaseModel):
    username: str
    password: str

# Simulated database of users
users_db = {
    "user1": {
        "username": "user1",
        "password": pwd_context.hash("password1"),
    },
    "user2": {
        "username": "user2",
        "password": pwd_context.hash("password2"),
    },
}

# Function to verify password
def verify_password(plain_password, hashed_password):
    return pwd_context.verify(plain_password, hashed_password)

# Function to generate JWT token
def create_access_token(data: dict, expires_delta: datetime.timedelta):
    to_encode = data.copy()
    expire = datetime.datetime.utcnow() + expires_delta
    to_encode.update({"exp": expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded_jwt

# Function to decode JWT token
def decode_access_token(token: str):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        return payload
    except jwt.ExpiredSignatureError:
        raise HTTPException(status_code=401, detail="Token has expired")
    except jwt.InvalidTokenError:
        raise HTTPException(status_code=401, detail="Invalid token")

# Route for user login
@app.post("/token")
async def login(user: User):
    if user.username not in users_db:
        raise HTTPException(status_code=401, detail="Invalid credentials")
    stored_user = users_db[user.username]
    if not verify_password(user.password, stored_user["password"]):
        raise HTTPException(status_code=401, detail="Invalid credentials")
    access_token = create_access_token(
        data={"sub": user.username},
        expires_delta=datetime.timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
    )
    return {"access_token": access_token, "token_type": "bearer"}

# Protected route
@app.get("/protected")
async def protected_route(token: str = Depends(oauth2_scheme)):
    payload = decode_access_token(token)
    username = payload["sub"]
    if username not in users_db:
        raise HTTPException(status_code=401, detail="Invalid credentials")
    return {"message": f"Welcome, {username}!"}
```

In this example, we're using FastAPI, Pydantic, passlib, and PyJWT to implement authorization and authentication. We define a User model, simulate a user database, and create routes for user login (/token) and a protected route (/protected). The verify_password function verifies the password hash, the create_access_token function generates a JWT token, and the decode_access_token function decodes and verifies the JWT token. The oauth2_scheme is used to retrieve the token from the request headers. Please note that this is a simplified example and may require further customization and security measures depending on your specific use case.