doing my pipe between 2 process

[elrik]

I want catch stdout and stderr of an child process and read them with
the parent process.
It's like popen4 but wihout shell commande.

something like that :

r, w = os.pipe()
input= os.fdopen(w)
pid=os.fork()
if pid: #parent
	while 1:
		input.read()
else:
	os.dup2(w, sys.stdout.fileno())
	print 'exemple'
	sys.stdout.flush()

but il doesn't work!
How can i make a write AND read file descriptor ?
Thanks