read stdout/stderr without blocking

[Jacek Popławski]

Popen from subprocess module gives me access to stdout, so I can read 
it. Problem is, that I don't know how much data is available... How can 
I read it without blocking my program?

example:
--------------------------------------------------------------------
import subprocess
import time

command="ls -l -R /"

p=subprocess.Popen(command,shell=True,stdout=subprocess.PIPE,stderr=subprocess.PIPE)

while (p.poll()==None):
         print "."
         r=p.stdout.read()
--------------------------------------------------------------------

when you comment out read() - you will notice that loop is working, with 
read() loop is blocked
Of course I don't need to read() inside loop, but... if output is very 
long (like from "make") and I don't read from stdout - command will 
block itself! I tried to increase bufsize, but it didn't help.

Is there a way to read only available data from stdout/stderr?
Is there a way to not block Popen command without reading stdout/stderr?