[Python] Getting XMLHTTPrequest CORS error in python

I'm building a sample app in Flutter calling Python backend. Whenever flutter call the backend URL, I receive XMLHTTPRequest error. I'll be grateful if someone can help.

My flutter code is as follow:

Future<void> fetchSubjects() async {
var url = Uri.parse('http://localhost:8000/subjects');
try {
final response = await http.get(
url,
headers: {
"Access-Control-Allow-Origin": "*",
"Access-Control-Allow-Headers":
"Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers",
},
);


Python code is as follow:

class S (BaseHTTPRequestHandler):
def dPTIONS(self):
print("OPTIONS request received")
# Handle preflight requests
self.send_response(200)
self.send_header('Access-Control-Allow-Origin', '*')
self.send_header('Access-Control-Allow-Methods', 'GET, POST')
self.send_header('Access-Control-Allow-Headers', 'Content-Type')
self.send_header("Access-Control-Allow-Headers", "Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers")
self.end_headers()

def do_GET(self):
self.send_header('Access-Control-Allow-Origin', '*')
self.send_response(200)
self.end_headers()
self.wfile.write(b'Hello, World!')

def run(server_class=HTTPServer, handler_class=S, port=8000):
server_address = ('', port)
httpd = server_class(server_address, handler_class)
fetchSubjects()
print(f'Starting server on port {port}...')
httpd.serve_forever()
if __name__ == '__main__':
run()


i tried adding CORS headers both in Flutter and in Python but still getting the error

The following error is received when I run the above code. enter image description here

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