Intercepting stdout of a subprocess while it is running
popen, process, python, stdout, subprocess
Solution
As Charles already mentioned, the problem is buffering. I ran in to a similar problem when writing some modules for SNMPd, and solved it by replacing stdout with an auto-flushing version.
I used the following code, inspired by some posts on ActiveState:
class FlushFile(object):
"""Write-only flushing wrapper for file-type objects."""
def __init__(self, f):
self.f = f
def write(self, x):
self.f.write(x)
self.f.flush()
# Replace stdout with an automatically flushing version
sys.stdout = FlushFile(sys.__stdout__)
Problem
If this is my subprocess: ``` import time, sys for i in range(200): sys.stdout.write( 'reading %i\n'%i ) time.sleep(.02) ``` And this is the script controlling and modifying the output of the subprocess: ``` import subprocess, time, sys print 'starting' proc = subprocess.Popen( 'c:/test_apps/testcr.py', shell=True, stdin=subprocess.PIPE, stdout=subprocess.PIPE ) print 'process created' while True: #next_line = proc.communicate()[0] next_line = proc.stdout.readline() if next_line == '' and proc.poll() != None: break sys.stdout.write(next_line) sys.stdout.flush() print 'done' ``` Why is `readline` and `communicate` waiting until the process is done running? Is there a simple way to pass (and modify) the subprocess' stdout real-time? I'm on Windows XP.