os.popen in unbuffered mode?

[Jonathan Mackenzie]

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!