fastapi判断用户是否第一次访问

要判断用户是否第一次访问，可以为每个用户设置一个唯一的标识符，例如使用浏览器中的cookie或者在服务器端使用session。使用FastAPI，可以通过以下方式获取和设置cookie和session：

获取cookie：

from fastapi import FastAPI, Request

app = FastAPI()

@app.get("/")
async def read_root(request: Request):
    cookie_value = request.cookies.get("user_id")
    if cookie_value:
        return {"message": "Welcome back!"}
    else:
        return {"message": "Welcome for the first time!"}

设置cookie：

from fastapi import FastAPI, Response

app = FastAPI()

@app.get("/")
async def read_root(response: Response):
    response.set_cookie(key="user_id", value="unique-user-id")
    return {"message": "Cookie set!"}

设置session：

from fastapi import FastAPI, Request, Response, Depends
from fastapi.security import HTTPBasic, HTTPBasicCredentials
from fastapi.responses import RedirectResponse
from fastapi.templating import Jinja2Templates
from typing import Optional

app = FastAPI()

security = HTTPBasic()
templates = Jinja2Templates(directory="templates")

fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "fakehashedsecret",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Wonderson",
        "email": "alice@example.com",
        "hashed_password": "fakehashedsecret2",
        "disabled": True,
    },
}

def get_current_user(session_token: Optional[str] = None):
    if not session_token:
        return None
    username = session_token.split(":")[0]
    user = fake_users_db.get(username)
    if user:
        return user
    return None

@app.get("/")
async def read_root(request: Request, current_user: Optional[dict] = Depends(get_current_user)):
    if current_user:
        return {"message": "Welcome back, " + current_user["full_name"] + "!"}
    else:
        return {"message": "Welcome for the first time!"}

@app.post("/login")
async def login(response: Response, credentials: HTTPBasicCredentials = Depends(security)):
    correct_username = credentials.username in fake_users_db
    correct_password = False
    if correct_username:
        user = fake_users_db[credentials.username]
        correct_password = credentials.password == user["hashed_password"]
    if not (correct_username and correct_password):
        return {"error": "Incorrect username or password", "response": response}
    session_token = credentials.username + ":" + credentials.password
    response.set_cookie(key="session_token", value=session_token)
    return RedirectResponse(url="/")

@app.post("/logout")
async def logout(response: Response):
    response.delete_cookie(key="session_token")
    return RedirectResponse(url="/")

在这个示例中，如果用户登录成功，会设置一个名为session_token的cookie，其中保存了用户名和密码的组合。然后在每次请求时，都会检查该cookie是否存在，并且使用该cookie中的信息来识别当前用户。