Python: execute a long script without block the main process

Question

I'm using GNU/Linux and python 2.7.3, I'm new on it. I'm trying to execute a long installed system process like ffmpeg using the Popen() command to avoid blocking the main python process. I can't make it work.

My first attempt was to use threads, but it is a bit complicated, because the graphical interface doesn't work properly (and I suppose there has to be a better way)

Then I tried fork, but it is like killing flies with tanks.

Now I'm trying to use Popen, I saw a lot of references to its use in the internet, but I'm not running it properly I think. Maybe I'm misunderstanding the examples.

I've tried:

##command is ffmpeg like that works ok
p = subprocess.Popen(command, shell=True)

I've tried also:

p = subprocess.Popen(command,stdin=PIPE, shell=True)

Usually I get an error or a sequentcial behavior, the command ends and then the main program resumes. What am I missing?

The command part:

    global vidSource
    global srtSource
    global done
    size = "3"

    font = "/usr/share/fonts/truetype/freefont/FreeSerif.ttf"
    command = 'mencoder ' + '"' + vidSource + '"' + " -oac copy -ovc lavc -lavdopts threads=2 -sub " + "'" + srtSource + "'" + " -subcp -utf8 -font "+ '"'+ font+ '"' + " -subfont-text-scale " + size + " -o " + '"' + vidSource + '2'+'"'

What error are you getting? You do know, Popen uses fork() (on linux) for creating new processes... If you didn't want to fork for some reason, Popen is not a viable solution. — tMC
– tMC, Commented Sep 26, 2012 at 14:10
Sorry, i forget to specify. I've tried to reproduce the error, but i get with the two comands i post a sequencial behaviour. :( — ferbuntu
– ferbuntu, Commented Sep 26, 2012 at 14:15

Pierre GM · Accepted Answer · 2012-09-26 14:29:39Z

2

You should be on a good track. Just use

p = subprocess.Popen(command)

You can check whether the process has ended with a p.poll, as described in the documentation

Note that using shell=True is not recommended (for security reasons), so you should try not to use it unless you have a very good reason.

As described in the documentation, your command must be a list of arguments. If you build a long string, you should use shlex.split(command) to transform it into a string while preserving the " and forth (of course, you need a import shlex in your module).

edited Sep 26, 2012 at 14:29

answered Sep 26, 2012 at 14:16

Pierre GM

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5 Comments

ferbuntu Over a year ago

I get an error: Traceback (most recent call last): File "/home/fer/myapp/myapp/MyappWindow.py", line 129, in on_addSubtitle_clicked p = subprocess.Popen(command) File "/usr/lib/python2.7/subprocess.py", line 679, in init errread, errwrite) File "/usr/lib/python2.7/subprocess.py", line 1249, in _execute_child raise child_exception OSError: [Errno 2] No such file or directory This is the only way i get an error, so i think the command is oK. Thanks for the help!

tMC Over a year ago

No such file or directory.. means you are trying to access/execute something that doesn't exist

ferbuntu Over a year ago

command = 'mencoder ' + '"' + vidSource + '"' + " -oac copy -ovc lavc -lavdopts threads=2 -sub " + "'" + srtSource + "'" + " -subcp -utf8 -font "+ '"'+ font+ '"' + " -subfont-text-scale " + size + " -o " + '"' + vidSource + '2'+'"' Where the vid sources are passed by user. I have tested it with a os.system call and are OK for it.

Pierre GM Over a year ago

@ferbuntu: please post the commands in your question with the proper formatting, it's gettng hard to read them in comments.

mgilson Over a year ago

@PierreGM -- The problem here is obvious. OP is passing a string to Popen which is being interpretted as executable to find. But, there is not executable named mencoder ".... Op needs to use p = subprocess.Popen(shlex.split(command)) (or I suppose use shell=True)

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