OK, so Guido told me at one point that 2.0 was supposed to have included changes to the way buffering of I/O to subprocesses is handled. I've installed 2.0, and I need to know how to make the following program work:

import os, sys

(child_stdin, child_stdout) = os.popen2("/usr/bin/rev", "t", 1); print "* Child process spawned"

while 1: line = raw_input(">"); child_stdin.write(line); child_stdin.flush() print "* Line written to child process" transformed = child_stdout.readline(); child_stdout.flush() print "* Line read back from child process." sys.stdout.write(transformed)

If anybody can tell me what the right magic is here, I'll write up a patch for the library docs explaining it and including the corrected example. This has to be a FAQ.

First, raw_input() strips the newline, so you should change the second line of the loop body to child_stdin.write(line+'\n'); child_stdin.flush() Second, with "cat" instead of "/usr/bin/rev", this works fine for me, so your logic is fine. The trick seems to be to get the subprocess not to buffer its output (or input). There's nothing that Python can do for you here! Unfortunately, most programs use stdio for all their console I/O, and stdio automatically buffers its output when connected to a pipe... :-( If you absolutely, positively have to do this with a program that won't unbuffer its output, your only resort would be to use pseudo ttys -- but that's quite a different story... :-( --Guido van Rossum (home page: http://www.python.org/~guido/)