[Python] CORS issue with fastAPI despite using a specific middleware

I have a CORS issue with this code from localhost:3000. I have checked the fastAPI documentation and add some code to tackle the issue. But there is no change and I have always the error.

Here is my code :

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=["*"],
)
app = FastAPI()
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


Here is the warning error :

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.


localhost:3000 is the client, localhost:4000 the server.

How to adresse this issue ?

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