How do you create a daemon in Python?


Answers

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.

Question

Searching on Google reveals x2 code snippets. The first result is to this code recipe which has a lot of documentation and explanation, along with some useful discussion underneath.

However, another code sample, whilst not containing so much documentation, includes sample code for passing commands such as start, stop and restart. It also creates a PID file which can be handy for checking if the daemon is already running etc.

These samples both explain how to create the daemon. Are there any additional things that need to be considered? Is one sample better than the other, and why?




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.



This function will transform an application to a daemon:

import sys
import os

def daemonize():
    try:
        pid = os.fork()
        if pid > 0:
            # exit first parent
            sys.exit(0)
    except OSError as err:
        sys.stderr.write('_Fork #1 failed: {0}\n'.format(err))
        sys.exit(1)
    # decouple from parent environment
    os.chdir('/')
    os.setsid()
    os.umask(0)
    # do second fork
    try:
        pid = os.fork()
        if pid > 0:
            # exit from second parent
            sys.exit(0)
    except OSError as err:
        sys.stderr.write('_Fork #2 failed: {0}\n'.format(err))
        sys.exit(1)
    # redirect standard file descriptors
    sys.stdout.flush()
    sys.stderr.flush()
    si = open(os.devnull, 'r')
    so = open(os.devnull, 'w')
    se = open(os.devnull, 'w')
    os.dup2(si.fileno(), sys.stdin.fileno())
    os.dup2(so.fileno(), sys.stdout.fileno())
    os.dup2(se.fileno(), sys.stderr.fileno())



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.

-Orby




I modified a few lines in Sander Marechal's code sample (mentioned by @JeffBauer in the accepted answer) to add a quit() method that gets executed before the daemon is stopped. This is sometimes very useful.

Here it is.

Note: I don't use the "python-daemon" module because the documentation is still missing (see also many other SO questions) and is rather obscure (how to start/stop properly a daemon from command line with this module?)




since python-daemon has not yet supported python 3.x, and from what can be read on the mailing list, it may never will, i have written a new implementation of PEP 3143: pep3143daemon

pep3143daemon should support at least python 2.6, 2.7 and 3.x

It also contains a PidFile class.

The library only depends on the standard library and on the six module.

It can be used as a drop in replacement for python-daemon.

Here is the documentation.




The easiest way to create daemon with Python is to use the Twisted event-driven framework. It handles all of the stuff necessary for daemonization for you. It uses the Reactor Pattern to handle concurrent requests.