Disable console output from subprocess.Popen in Python

Question

I run Python 2.5 on Windows, and somewhere in the code I have

subprocess.Popen("taskkill /PID " + str(p.pid))

to kill IE window by pid. The problem is that without setting up piping in Popen I still get output to console - SUCCESS: The process with PID 2068 has been terminated. I debugged it to CreateProcess in subprocess.py, but can't go from there.

Anyone knows how to disable this?

Solution 1

fh = open("NUL","w")
subprocess.Popen("taskkill /PID " + str(p.pid), stdout = fh, stderr = fh)
fh.close()

Solution 2

import os
from subprocess import check_call, STDOUT

DEVNULL = open(os.devnull, 'wb')
try:
    check_call(("taskkill", "/PID", str(p.pid)), stdout=DEVNULL, stderr=STDOUT)
finally:
    DEVNULL.close()

I always pass in tuples to subprocess as it saves me worrying about escaping. check_call ensures (a) the subprocess has finished before the pipe closes, and (b) a failure in the called process is not ignored. Finally, os.devnull is the standard, cross-platform way of saying NUL in Python 2.4+.

Note that in Py3K, subprocess provides DEVNULL for you, so you can just write:

from subprocess import check_call, DEVNULL, STDOUT

check_call(("taskkill", "/PID", str(p.pid)), stdout=DEVNULL, stderr=STDOUT)

Comments

[Mark]

I tried that first, for some reason it doesn't parse correctly. >>> ERROR: Invalid Argument/Option - '>'. Type "TASKKILL /?" for usage. That works on the cmd line though.

[Alice Purcell]

I think there's a race condition there — you may close the pipe before your subprocess has finished and cause it to terminate early.

[orip]

@chrispy - you're correct, I think there should be a .communicate() in there

[Harvey]

@chrispy,@orip: actually, isn't the answer to add .wait() to the subprocess line?

[nbarraille]

Just a silly question: I want to do the exact same thing, and I don't understand why stdout=None prevent the thread to display stuff...

[Melroy van den Berg]

You can better use: os.devnull instead of "NUL", because that is cross-platform. Windows uses: 'nul' and Linux uses: '/dev/null'.