FastAPI配置CORSMiddleware后仍存在CORS跨域问题及后续405、422错误的排查与解决咨询

我在本地localhost:3000的前端调用FastAPI后端（localhost:4000）时遇到了CORS跨域问题，已经参照FastAPI文档添加了相关代码来处理，但问题依然存在，一直收到跨域错误提示。

最初的后端代码： #

import httpx
from fastapi import FastAPI
from pydantic import BaseModel
from starlette.middleware.cors import CORSMiddleware

app = FastAPI()

origins = [
    "http://localhost:3000"
]

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

msgs=dict()

class Body(BaseModel):
    title: str

class Msg(BaseModel):
    type: str
    content: dict
@app.post("/posts/")
async def posts(body: Body):
    number= len(msgs)+1
    title = body
    msgs[number]= {
            "type": "PostCreated",
            "data": {
                "id": number,
                "title": title.title
            }
        }
    api_url = "http://localhost:4005/events"
    async with httpx.AsyncClient() as client:
        response = await client.post(api_url, json={
            "type": "PostCreated",
            "data": {
                number, title
            }
        })

    return msgs

@app.post("/events/")
async def event_monitoring(msg: Msg)  :
    print(msg.type)
@app.get("/posts/")
async def root():
    return  msgs

收到的跨域错误提示： #

Access to XMLHttpRequest at 'http://localhost:4000/posts/' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

编辑1：服务器日志返回错误

"OPTIONS /posts HTTP/1.1" 405 Method Not Allowed

明明已经设置了allow_methods=["*"]允许所有请求方法，还是出现了这个405错误。

编辑2：调整代码后CORS问题解决，但出现422错误
修改后的代码如下（主要修正了事件推送的JSON格式和API路径）：

import httpx
from fastapi import FastAPI
from pydantic import BaseModel
from starlette.middleware.cors import CORSMiddleware

app = FastAPI()

origins = [
    "http://localhost:3000"
]

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

msgs=dict()

class Body(BaseModel):
    title: str

class Msg(BaseModel):
    type: str
    content: dict
@app.post("/posts/")
async def posts(body: Body):
    number= len(msgs)+1
    title = body
    msgs[number]= {
            "type": "PostCreated",
            "data": {
                "id": number,
                "title": title.title
            }
        }
    api_url = "http://localhost:4005/events/"
    async with httpx.AsyncClient() as client:
        response = await client.post(api_url, json={
            "type": "PostCreated",
            "data": {
                "id": number,
                "title": title.title
            }
        })

    return msgs

@app.post("/events/")
async def event_monitoring(msg: Msg)  :
    print(msg.type)
@app.get("/posts/")
async def root():
    return  msgs

编辑3：最终解决422错误
发现问题出在Msg类的定义上，原来的字段名和实际接收的JSON字段不匹配，修正后问题彻底解决：

class Msg(BaseModel):
    type: str
    data: dict

备注：内容来源于stack exchange，提问作者EagleStar