Sending 404 Response Code in HTTPD

Python Help
1

I am trying to send a 404 response code in my do_GET but all I get is a blank page. How does one send a response code? TIA

if (os.path.isfile(fn)):
    self.send_response(200)
    self.send_header('Content-type',ctype)
    self.end_headers()
    fh=open(fn,'rb')
    string=fh.read()
    self.wfile.write(string)
else:
    self.send_response(404,message='Not Found')
    self.end_headers()