CORS with Flask

MDN has a great overview of CORS and when it matters. I was trying to run a svelte app using npm run dev which by default launches on http://localhost:5000. I made a request from svelte using the fetch API to a Python Flask server that ran on http://localhost:1234

However, the request failed with the following error

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:1234/dimensions. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

Basically, the browser will only allow reading remote resources when the Allow-Control-Allow-Origin is as a part of the server response.

While I could have manually added this header, Flask has a simple extension that lets us add CORS headers.

from flask import Flask, send_from_directory
from flask_cors import CORS, cross_origin
from time import sleep

app = Flask(__name__)

def get_dimenisons():
    return { "x": 7, "y": 6 }, port=1234)

CORS(app) enables CORS for all domains on routes.

However, one could also get more specific by annotating specific routes using @cross_origin()