Using os.popen("command", 'r', 0) on Windows 2000 (python 2.2.1) and I get: pa = os.popen(progStr, 'r', 0) ValueError: popen() arg 3 must be -1 Can anyone please tell me how to put popen into unbuffered mode? I'm looking to get real-time returns from the subprocess, which is writing to stdout. Thanks!