FastAPI + CORS: A Quick Solution to Cross-Origin Resource Sharing Issues

Cross-origin issues occur when a frontend requests backend APIs from different domains, ports, or protocols, which are blocked by the browser's same-origin policy. By default, FastAPI does not handle cross-origin requests and requires the CORS middleware to resolve this. The core solution involves adding `CORSMiddleware` to FastAPI, with key parameters including: `allow_origins` (allowed frontend domains, use `["*"]` for development and specify specific domains in production), `allow_credentials` (whether to allow cookies to be carried across origins), `allow_methods` (allowed HTTP methods), and `allow_headers` (allowed request headers). It is crucial to avoid `allow_origins=["*"]` in production environments, as this should be restricted to specific domains. When allowing credentials, `allow_origins` must be explicitly specified. After configuration, the frontend can request backend APIs normally, such as the example where `fetch("http://localhost:8000/api/hello")` returns data. In summary, configuring the CORS middleware safely resolves cross-origin issues, offering flexibility in development while requiring strict parameter restrictions in production.