I thought I was un-confusing sys.std* Python objects (such as idlelib.rpc.RPCProxy) from std* integer file descriptors, whereas you seem to say they are the same: "copies of the file descriptors (in particular, sys.std*)". The rest of your message seems to imply that subprocess.check_output should not work, but it does, at least on Windows. And subprocess *is* a recommended replacement for os.system.

Roger, do you have any thoughts on this issue?