Python: How to read stdout non blocking from another process?

During the runtime of a process I would like to read its stdout and write it to a file. Any attempt of mine however failed because no matter what I tried as soon as I tried reading from the stdout it blocked until the process finished.

Here is a snippet of what I am trying to do. (The first part is simply a python script that writes something to stdout.)

import subprocess

p = subprocess.Popen('python -c \'\
from time import sleep\n\
for i in range(3):\n\
    sleep(1)\n\
    print "Hello", i\
\'', shell = True, stdout = subprocess.PIPE)

while p.poll() == None:
    #read the stdout continuously
    pass

print "Done"

I know that there are multiple questions out there that deal with the same subject. However, none of the ones I found was able to answer my question.