read subprocess stdout line by line

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 and show some of it to the user. I thought the following would work, but the output doesn’t 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?

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Update:

If a sys.stdout.flush() is added to fake_utility.py, the code has the desired behavior in python 3.1. I’m using python 2.6. You would think that using proc.stdout.xreadlines() would work the same as py3k, but it doesn’t.

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Update 2:

Here is the minimal working code.

#fake_utility.py, just generates lots of output over time
import sys, time
for i in range(10):
   print i
   sys.stdout.flush()
   time.sleep(0.5)

#display out put line by line
import subprocess
proc = subprocess.Popen(['python','fake_utility.py'],stdout=subprocess.PIPE)
#works in python 3.0+
#for line in proc.stdout:
for line in iter(proc.stdout.readline,''):
   print line.rstrip()

Answer

I think the problem is with the statement for line in proc.stdout, which reads the entire input before iterating over it. The solution is to use readline() instead:

#filters output
import subprocess
proc = subprocess.Popen(['python','fake_utility.py'],stdout=subprocess.PIPE)
while True:
  line = proc.stdout.readline()
  if not line:
    break
  #the real code does filtering here
  print "test:", line.rstrip()

Of course you still have to deal with the subprocess’ buffering.

Note: according to the documentation the solution with an iterator should be equivalent to using readline(), except for the read-ahead buffer, but (or exactly because of this) the proposed change did produce different results for me (Python 2.5 on Windows XP).