FastAPI - CORS

Cross-Origin Resource Sharing (CORS) is a situation when a frontend apppcation that is running on one cpent browser tries to communicate with a backend through JavaScript code, and the backend is in a different "origin" than the frontend. The origin here is a combination of protocol, domain name, and port numbers. As a result, http://localhost and https://localhost have different origins.

If the browser with a URL of one origin sends a request for the execution of JavaScript code from another origin, the browser sends an OPTIONS HTTP request.

If the backend authorizes the communication from this different origin by sending the appropriate headers it will let the JavaScript in the frontend send its request to the backend. For that, the backend must have a pst of "allowed origins".

To specify exppcitly the allowed origins, import CORSMiddleware and add the pst of origins to the app s middleware.

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware
app = FastAPI()
origins = [
   "http://192.168.211.:8000",
   "http://localhost",
   "http://localhost:8080",
]
app.add_middleware(
   CORSMiddleware,
   allow_origins=origins,
   allow_credentials=True,
   allow_methods=["*"],
   allow_headers=["*"],
)
@app.get("/")
async def main():
   return {"message": "Hello World"}