Call a python subprocess as daemon and exit

Question

I'm using a pair of python programs, one of which should call the second.

But this should be done in a way that the first program makes the second one a daemon (or running in the background process), then exits, without waiting for the second program to end.

Is this possible in Python?

I've been looking at os.fork, subprocess module, but I'm quite confused as the correct way to achieve this...

Answer 1

You can use subprocess.Popen for this:

import subprocess

cmd = ['/usr/bin/python', '/path/to/my/second/pythonscript.py']
subprocess.Popen(cmd)

You might want to redirect stdout and stderr somewhere, you can do that by passing stdout=<file_obj> to the Popen constructor.

Answer 2

A "daemon" carries a more specific definition than just "background process". By default Popen will execute a subprocess and give control back to your program to continue running. However, that says nothing about dealing with signals (notably, SIGHUP if the user exits their shell before your background process finished) or disconnecting from stdin, stderr, stdout and any other file descriptors opened at the time you invoke Popen. If you truly mean that you want a daemon, you should use the python-daemon module, an implementation of PEP 3143. This takes care of the gross parts you don't want to think about.