python,raspberry-pi , Popen.kill() failing


Popen.kill() failing

Question:

Tag: python,raspberry-pi

Update 2: So I piped the output of stderr and it looks like when I include shell=True, i just get the help file for omx player (it lists all the command line switches and such). Is it possible that shell=True might not play nicely with omxplayer?

Update: I came across that link before but it failed on me so I moved on without digging deeper. After Tshepang suggested it again I looked into it further. I have two problems, and I'm hoping the first is caused by the second. The first problem is that when I include shell=True as an arg, the video never plays. If I don't include it, the video plays, but is not ever killed. Updated code below.

So I am trying to write a python app for my raspberry pi that plays a video on a loop (I came across Popen as a good way to accomplish this using OMXplayer) and then on keyboard interrupt, it kills that process and opens another process (playing a different video). My eventual goal is to be able to use vid1 as a sort of "screensaver" and have vid2 play when a user interacts with the system, but for now im simply trying to kill vid1 on keyboard input and running into quite the hard time doing it. I'm hoping someone can tell me where my code is falling down.

Forewarning that I'm extremely new to Python, and linux based systems in general, so if im doing this terribly wrong, please feel free to redirect me, but this seemed to be the fastest way to get there.

Here is my code as it stands:

import subprocess
import os
import signal

vid1 = ['omxplayer', '--loop', '/home/pi/Vids/2779832.mp4']

while True:
    #vid = subprocess.Popen(['omxplayer', '--loop',     '/home/pi/Vids/2779832.mp4'], stdout=subprocess.PIPE, shell=True,     preexec_fn=os.setsid)
    vid = subprocess.Popen(vid1, stdout=subprocess.PIPE, preexec_fn=os.setsid)
    print 'SID is: ', preexec_fn
    #vid = subprocess.Popen(['omxplayer', '--loop',     '/home/pi/Vids/2779832.mp4'])
    id = raw_input()
    if not id:
        break
    os.killpg(vid.pid, signal.SIGTERM)
    print "your input: ", id
print "While loop has exited"

Answer:

So I am trying to write a python app for my raspberry pi that plays a video on a loop (I came across Popen as a good way to accomplish this using OMXplayer) and then on keyboard interrupt, it kills that process and opens another process (playing a different video).

By default, SIGINT is propagated to all processes in the foreground process group, see "How Ctrl+C works". preexec_fn=os.setsid (or os.setpgrp) actually prevents it: use it only if you do not want omxplayer to receive Ctrl+C i.e., use it if you manually call os.killpg when you need to kill a process tree (assuming omxplayer children do not change their process group).

"keyboard interrupt" (sigint signal) is visible as KeyboardInterrupt exception in Python. Your code should catch it:

#!/usr/bin/env python
from subprocess import call, check_call

try:
   rc = call(['omxplayer', 'first file'])
except KeyboardInterrupt:
   check_call(['omxplayer', 'second file'])
else:
   if rc != 0:
      raise RuntimeError('omxplayer failed to play the first file, '
                         'return code: %d' % rc)

The above assumes that omxplayer exits on Ctrl+C.

You could see the help message due to several reasons e.g., omxplayer does not support --loop option (run it manually to check) or you mistakenly use shell=True and pass the command as a list: always pass the command as a single string if you need shell=True and in reverse: always (on POSIX) pass the command as a list of arguments if shell=False (default).


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