My python script uses subprocess to call a linux utility that is very noisy. I want to store all of the output to a log file, but only show some of it to the user. I thought the following would work, but the output does show up in my application until the utility has produced a significant amount of output.

#fake_utility.py, just generates lots of output over time
import time
i = 0
while True:
   print hex(i)*512
   i += 1
   time.sleep(0.5)

#filters output
import subprocess
proc = subprocess.Popen(['python','fake_utility.py'],stdout.subprocess.PIPE)
for line in proc.stdout:
   #the real code does filtering here
   print "test:", line.rstrip()

The behavior I really want is for the filter script to print each line as it is received from the subprocess. Sorta like what tee does but with python code.

What am I missing? Is this even possible?