How do you create a daemon in Python?

Question

Current solution

A reference implementation of PEP 3143 (Standard daemon process library) is now available as python-daemon.

Historical answer

Sander Marechal's code sample is superior to the original, which was originally posted in 2004. I once contributed a daemonizer for Pyro, but would probably use Sander's code if I had to do it over.

There are many fiddly things to take care of when becoming a well-behaved daemon process:

• prevent core dumps (many daemons run as root, and core dumps can contain sensitive information)

• behave correctly inside a chroot gaol

• set UID, GID, working directory, umask, and other process parameters appropriately for the use case

• relinquish elevated suid, sgid privileges

• close all open file descriptors, with exclusions depending on the use case

• behave correctly if started inside an already-detached context, such as init, inetd, etc.

• set up signal handlers for sensible daemon behaviour, but also with specific handlers determined by the use case

• redirect the standard streams stdin, stdout, stderr since a daemon process no longer has a controlling terminal

• handle a PID file as a cooperative advisory lock, which is a whole can of worms in itself with many contradictory but valid ways to behave

• allow proper cleanup when the process is terminated

• actually become a daemon process without leading to zombies

Some of these are standard, as described in canonical Unix literature (Advanced Programming in the UNIX Environment, by the late W. Richard Stevens, Addison-Wesley, 1992). Others, such as stream redirection and PID file handling, are conventional behaviour most daemon users would expect but that are less standardised.

All of these are covered by the PEP 3143 “Standard daemon process library” specification. The python-daemon reference implementation works on Python 2.7 or later, and Python 3.2 or later.

Here's my basic 'Howdy World' Python daemon that I start with, when I'm developing a new daemon application.

#!/usr/bin/python
import time
from daemon import runner

class App():
    def __init__(self):
        self.stdin_path = '/dev/null'
        self.stdout_path = '/dev/tty'
        self.stderr_path = '/dev/tty'
        self.pidfile_path =  '/tmp/foo.pid'
        self.pidfile_timeout = 5
    def run(self):
        while True:
            print("Howdy!  Gig'em!  Whoop!")
            time.sleep(10)

app = App()
daemon_runner = runner.DaemonRunner(app)
daemon_runner.do_action()

Note that you'll need the python-daemon library. You can install it by:

pip install python-daemon

Then just start it with ./howdy.py start, and stop it with ./howdy.py stop.

Note the python-daemon package which solves a lot of problems behind daemons out of the box.

Among other features it enables to (from Debian package description):

• Detach the process into its own process group.
• Set process environment appropriate for running inside a chroot.
• Renounce suid and sgid privileges.
• Close all open file descriptors.
• Change the working directory, uid, gid, and umask.
• Set appropriate signal handlers.
• Open new file descriptors for stdin, stdout, and stderr.
• Manage a specified PID lock file.
• Register cleanup functions for at-exit processing.

An alternative -- create a normal, non-daemonized Python program then externally daemonize it using supervisord. This can save a lot of headaches, and is *nix- and language-portable.

Probably not a direct answer to the question, but systemd can be used to run your application as a daemon. Here is an example:

[Unit]
Description=Python daemon
After=syslog.target
After=network.target

[Service]
Type=simple
User=<run as user>
Group=<run as group group>
ExecStart=/usr/bin/python <python script home>/script.py

# Give the script some time to startup
TimeoutSec=300

[Install]
WantedBy=multi-user.target

I prefer this method because a lot of the work is done for you, and then your daemon script behaves similarly to the rest of your system.