尝试使用子进程执行Python脚本（Django）

Question

I'm trying to execute a script using subprocess that I know works when I execute it by hand; the following is from my calling script:

# the command string we want to issue to ffmpeg.py to generate our ffmpeg command strings
        commandString = [
            'python',
            os.path.join(SCRIPT_DIR, 'ffmpeg.py'),
            '-i', os.path.join('/srv/nfsshare/transcode50', userFolder, directory, title),
            '-d', os.path.join('/srv/nfsshare/transcode50', userFolder, directory),
            '-r', request.POST['framerate'],
            '-p 2', '-f', ",".join(formats), '-t', ",".join(rasters)
        ]

        # call transcode50 script to generate condor_execute.py
        subprocess.call(' '.join(commandString) + ' > /srv/nfsshare/transcode50/output.txt', shell=True)

The actual script itself essentially will generate a list of command strings and output them to the console. I piped the output to a file called output.txt at the end of that command string to test this, as I'm running the Python code from Django and can't see the shell output in realtime, but when I examine the file each time, nothing is in there, and the side effect the called script also has (generating a Python file) doesn't take place. Therefore, I trust there is something I may or may not be considering using the subprocess module, and perhaps it's Django-specific?

Answer 1

Converting a list to a shell string using ' '.join(...) is risky because there may be something in the list (like a space in a file name) that needs shell escaping. You are better off just sticking with the command list and not the shell. You should also capture stderr which is where the good stuff is going to be. Finally use check_call and wrap the whole thing in an exception handler that logs execution failures.

try:
    commandString = [
        'python',
        os.path.join(SCRIPT_DIR, 'ffmpeg.py'),
        '-i', os.path.join('/srv/nfsshare/transcode50', userFolder, directory, title),
        '-d', os.path.join('/srv/nfsshare/transcode50', userFolder, directory),
        '-r', request.POST['framerate'],
        '-p 2', '-f', ",".join(formats), '-t', ",".join(rasters)
    ]

    # call transcode50 script to generate condor_execute.py
    subprocess.check_call(commandString, 
        stdout=open('/srv/nfsshare/transcode50/output.txt', 'w'),
        stderr=subprocess.STDOUT)

except Exception, e:
    # you can do fancier logging, but this is quick
    open('/tmp/test_exception.txt', 'w').write(str(e))
    raise