Python subprocess.Popen()

Question

Please go through the python code snippet below

import subprocess  
child = subprocess.Popen("python ./myprog.py ", shell=True)  
print "Hello"

Where myprog.py is simply :
print "Hello,myprog"

The output I get is :

Hello
Hello,myprog

My question is whether the child process starts running parallel to the parent process or does the child waits for the parent process to finish off,as the above output suggests?

Answer 1

According to python docs http://docs.python.org/library/subprocess.html

subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False)
Run the command described by args. Wait for command to complete, then return the returncode attribute.

So it appears that the spawn process is a child process for the python script itself.

Answer 2

disconnecting a subprocess is the same as creating a daemon. See http://code.activestate.com/recipes/278731-creating-a-daemon-the-python-way/

As to your original question, the subprocess at least starts executing before the next statement. Note that you will need to process the stdout of that subprocess to actually get it to print.

Answer 3

To my understanding, it is launched as another process in the same fashion as a Unix fork() or whatever the Windows equivalent is.

And then there's this: http://docs.python.org/library/subprocess.html#subprocess.Popen.wait

Observe:

>>> p = Popen(["ls", "-hal"]);sleep (5);print "Word"
total 12M
drwxr-xr-x 51 ishpeck users   4.0K Nov 14 19:58 .
drwxr-xr-x  7 root    root    4.0K Jun 25 04:30 ..
drwxr-xr-x  2 ishpeck users   4.0K Jun  6 00:46 abcde.b00b030d
drwx------  2 ishpeck users   4.0K Nov 13 13:57 .AbiSuite
drwx------  3 ishpeck users   4.0K May 29 20:14 .adobe
-rw-r--r--  1 ishpeck archive   55 Jun 29 07:18 .apvlvinfo
drwxr-xr-x  2 ishpeck archive 4.0K Jul 28 23:16 .archive
-rw-r--r--  1 ishpeck archive  180 Nov 14 18:23 .aspell.en.prepl
-rw-r--r--  1 ishpeck archive  250 Nov 14 18:23 .aspell.en.pws
-rw-r--r--  1 ishpeck users    152 Jun  6 00:45 .asunder
-rw-------  1 ishpeck users   9.0K Nov 14 20:06 .bash_history
-rw-r--r--  1 ishpeck users   2.2K Oct 22 08:54 .bashrc
drwx------  3 ishpeck users   4.0K Sep 28 08:23 .bitcoin
drwx------  3 ishpeck archive 4.0K Jul 30 22:44 .Broken Rules
drwx------ 10 ishpeck users   4.0K Aug 16 16:37 .cache
drwx------  2 ishpeck users   4.0K May 30 03:42 .cmus
drwxr-xr-x 11 ishpeck users   4.0K Nov  6 16:42 .codelite
drwx------ 23 ishpeck users   4.0K Sep 22 00:18 .config
-rw-r--r--  1 ishpeck users   2.6K Jun  5 17:21 .conkyrc
drwx------  3 ishpeck users   4.0K May 28 08:29 .dbus
drwx------ 13 ishpeck users   4.0K Sep 27 21:25 dl
drwxr-xr-x 29 ishpeck users   4.0K Nov 12 00:00 docs
-rw-r--r--  1 ishpeck archive 1000 Nov 11 06:26 .emacs
-rw-r--r--  1 ishpeck archive 1000 Nov 11 06:25 .emacs~
-rw-r--r--  1 ishpeck archive  511 Nov 10 11:14 .emacs-backup
drwx------  6 ishpeck users   4.0K Nov 10 11:22 .emacs.d
drwxr-xr-x  3 ishpeck archive 4.0K Jun 17 04:26 .Eterm
-rw-r--r--  1 ishpeck users    105 Nov 14 20:00 .fehbg
-rw-------  1 ishpeck archive    9 Nov 13 22:10 .fetchmail.pid
-rw-------  1 ishpeck archive  323 Oct 30 08:10 .fetchmailrc
drwxr-xr-x  2 ishpeck users   4.0K Nov  2 00:43 .fontconfig
drwx------  4 ishpeck users   4.0K Nov 13 22:10 .gconf
drwx------  2 ishpeck users   4.0K Oct  1 20:50 .gconfd
-rw-r--r--  1 ishpeck archive 127K Nov  7 10:19 geany-colors.tar.gz
drwxr-xr-x  2 ishpeck users   4.0K May 31 05:43 .ghc
drwxr-xr-x 22 ishpeck archive 4.0K Nov 14 18:24 .gimp-2.6
-rw-r--r--  1 ishpeck users     75 May 31 12:59 .gitconfig
-rw-r--r--  1 ishpeck archive  893 Oct 12 20:03 .gitk
drwx------  3 ishpeck archive 4.0K Jul 30 19:46 .gnome
drwx------  3 ishpeck users   4.0K Jul  1 13:37 .gnome2
drwx------  2 ishpeck archive 4.0K Jul  1 13:37 .gnome2_private
drwxr-xr-x  2 ishpeck users   4.0K Nov  2 19:05 .gstreamer-0.10
-rw-------  1 ishpeck users    234 Nov  7 20:36 .gtk-bookmarks
-rw-r--r--  1 ishpeck users    164 Jun  1 04:10 .gtkrc-2.0
drwx------  2 ishpeck users   4.0K May 30 03:35 .gvfs
drwxr-xr-x  3 ishpeck users   4.0K May 28 15:51 .icons
drwxr-xr-x  2 ishpeck archive 4.0K Aug 11 06:59 .idlerc
drwx------  2 ishpeck archive 4.0K Jul 16 19:36 .irssi
-rw-------  1 ishpeck users    846 Nov 11 20:09 .lesshst
drwx------  3 ishpeck users   4.0K Nov 11 22:35 .liferea_1.6
drwx------  3 ishpeck users   4.0K May 28 06:12 .local
-rw-r--r--  1 ishpeck users    842 Jun  5 16:34 lynx_bookmarks.html
-rw-------  1 ishpeck users   3.3K Jun 11 04:14 Lynx.trace
drwx------  3 ishpeck users   4.0K May 29 20:14 .macromedia
drwx------  2 ishpeck users   4.0K Sep  2 19:42 Mail
drwxr-xr-x  5 ishpeck users   4.0K Nov  7 07:47 media
-rw-r--r--  1 ishpeck archive 3.8K Jul 16 22:56 .mrxvtrc
drwxr-xr-x  2 ishpeck archive 4.0K Sep 14 08:12 .mutt
-rw-r--r--  1 ishpeck archive 3.3K Oct 14 06:32 .muttrc
-rw-r--r--  1 ishpeck archive 1.3K Jul 27 22:36 .nvidia-settings-rc
drwx------  3 ishpeck users   4.0K May 28 08:35 .pki
-rw-------  1 ishpeck archive    0 Nov  9 07:24 postponed
drwxr-xr-x 12 ishpeck users   4.0K Sep 28 08:08 project
drwx------  2 ishpeck archive 4.0K Nov 14 18:07 .pulse
-rw-------  1 ishpeck users    256 May 31 03:52 .pulse-cookie
drwx------  6 ishpeck users   4.0K Nov 14 20:15 .purple
-rw-r--r--  1 ishpeck users    12K Nov  2 07:06 .pypanelrc
drwxr-xr-x  3 ishpeck users   4.0K Jun 11 02:29 .q3a
-rw-r--r--  1 ishpeck archive 1.9K Aug 23 17:11 .recently-used
drwxr-xr-x  2 ishpeck users   4.0K Nov  2 07:55 .sbin
-    rw-------  1 ishpeck archive  11M Nov 14 19:52 sent
-rw-r--r--  1 ishpeck archive   61 Jul  2 21:44 .signature
drwxr-xr-x  3 ishpeck archive 4.0K Nov  5 07:43 .slime
drwx------  2 ishpeck users   4.0K Oct 31 22:46 .ssh
drwxr-xr-x  3 ishpeck archive 4.0K Aug  6 16:49 .steelstorm
drwxr-xr-x  3 ishpeck archive 4.0K Jul 28 23:16 .subversion
-rw-r--r--  1 ishpeck archive  273 Jun 18 02:47 .synergy.conf
drwxr-xr-x  9 ishpeck users   4.0K Jun  1 04:10 .themes
drwx------  3 ishpeck users   4.0K May 28 09:39 .thumbnails
drwxr-xr-x  2 ishpeck users   4.0K Oct 25 06:36 .vim
-rw-------  1 ishpeck archive  19K Nov 14 19:52 .viminfo
-rw-r--r--  1 ishpeck users     65 Jun 17 22:30 .vimrc
drwxr-xr-x  2 ishpeck archive 4.0K Nov 13 23:26 .VirtualBox
drwxr-xr-x  3 ishpeck archive 4.0K Jul 30 19:52 .vvvvvv
-rw-------  1 ishpeck archive  101 Aug 17 05:51 .Xauthority
-rw-r--r--  1 ishpeck archive  134 Aug 19 06:32 .Xdefaults
-rw-r--r--  1 ishpeck users    120 Sep 26 21:19 .xinitrc
-rw-r--r--  1 ishpeck archive 7.6K Oct 13 07:23 .xscreensaver
Word

So, in essence, yes, it's running in parallel.