Buffering problem using subprocess module

[Dr. Who]

I am using the subprocess module in 2.4. Here's the fragment:

bufcaller.py:
import sys, subprocess
proc = subprocess.Popen('python bufcallee.py', bufsize=0, shell=True,
stdout=subprocess.PIPE)
for line in proc.stdout:
sys.stdout.write(line)

bufcallee.py:
import time
print 'START'
time.sleep(10)
print 'STOP'

Although the documentation says that the output should be unbuffered
(bufsize=0) the program (bufcaller) pauses for 10 seconds and then
prints START immediately followed by 'STOP' rather than pausing 10
seconds in between them. Note that I made bufcallee a Python script
for ease of the example but in the real-world problem I am trying to
solve it is simply an executable.

Any ideas?
Jeff

[Dr. Who]

Unfortunatley that doesn't help. Even when callee is started using the
-u, I get the same behavior.