I wanted to run a shell command in python without knowing if the shell command is going to exit within reasonable time (adplay that was, sometimes it simply hangs).

Update: the “task” module of Rob Hooft seems to solve this exact problem. At the time I wrote this, the python.net website was down. I leave my solution here just for archive purpose.

def timeout_command(command, timeout):
  """call shell-command and either return its output or kill it
  if it doesn't normally exit within timeout seconds and return None"""
  import subprocess, datetime, os, time, signal
  start = datetime.datetime.now()
  process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
  while process.poll() is None:
    now = datetime.datetime.now()
    if (now - start).seconds> timeout:
      os.kill(process.pid, signal.SIGKILL)
      os.waitpid(-1, os.WNOHANG)
      return None
  return process.stdout.read()


>>> output = timeout_command(["sleep", "10"], 2)
>>> output = timeout_command(["sleep", "1"], 2)

The process can be killed when it has run for too long (the os.waitpid waits for the kill to end and avoids defunct-processes) and furthermore the Popen’ed process’ printed is caught and returned if it doesn’t timeout. However, subprocess.Popen is called with a list as argument. That means, that the command isn’t passed to a shell and furthermore you can just call one command with options, nothing more.