How do you create a daemon in Python?

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.