python run external program and continue its execution indepentently

Question

How can i run an external program, let's say "Firefox", from my python script and make sure that its process will remain alive after the termination of my python script? I want to make it crossplatform if it's doable.

http://stackoverflow.com/questions/204017/how-do-i-execute-a-program-from-python-os-system-fails-due-to-spaces-in-path — jonjohnson, Feb 16 '14 at 00:49
@jonjohnson: How does that answer this question, or even related to it? The question there is about a quoting problem with `os.system`, and the answer is to use `subprocess.call`, which will block until the child process finishes… — abarnert, Feb 16 '14 at 00:54
related: [Popen waiting for child process even when the immediate child has terminated](http://stackoverflow.com/q/13243807/4279) — jfs, Feb 18 '14 at 09:32

abarnert · Accepted Answer · 2014-02-16T00:42:19.490

There is no cross-platform way to do this with just the stdlib. However, if you write code for POSIX and for Windows, that's usually good enough, right?

On Windows, you want to pass a creationflags argument. Read the docs (both there and at MSDN) and decide whether you want a console-detached process, a new-console process, or a new-process-group process, then use the appropriate flag. You may also want to set some of the flags in startupinfo; again, MSDN will tell you what they mean.

On POSIX, if you just want the simplest behavior, and you're using 3.2+, you want to pass start_new_session=True. In earlier Python versions, or for other cases, you want to pass a preexec_fn that allows you to do whatever daemonization you want. That could be as little as os.setsid() (what start_new_session does), or a whole lot more. See PEP 3143 -- Standard daemon process library for a discussion of all of the different things you might want to do here.

So, the simplest version is:

def launch_in_background(args):
    try:
        subprocess.CREATE_NEW_PROCESS_GROUP
    except AttributeError:
        # not Windows, so assume POSIX; if not, we'll get a usable exception
        p = subprocess.Popen(args, start_new_session=True)
    else:
        # Windows
        p = subprocess.Popen(args, creationflags=subprocess.CREATE_NEW_PROCESS_GROUP)

If you're willing to go outside the stdlib, there are dozens of "shell-type functionality" libraries out there, many of which have some kind of "detach" functionality. Just search shell, cli, or subprocess at PyPI and find the one you like best.

python run external program and continue its execution indepentently

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