A simple daemon in Python

daemon.py

"""
    daemon
    ~~~~~~

    A simple daemon.

    :copyright: (c) 2015 by Shipeng Feng.
    :license: BSD.
"""
import os
import sys
import atexit


class Daemon(object):
    def __init__(self, pidfile):
        self.pidfile = pidfile

    def start(self):
        """Start the daemon."""
        # Check pidfile
        try:
            pf = open(self.pidfile, 'r')
            pid = int(pf.read().strip())
            pf.close()
        except IOError:
            pid = None
        if pid is not None:
            raise RuntimeError('pidfile %s already exist, does the daemon '
                               'already run with pid %s?' %
                               (self.pidfile, pid))
        # Daemonize
        if os.fork() > 0:
            sys.exit(0)
        os.setsid()
        if os.fork() > 0:
            sys.exit(0)
        os.chdir('/')
        os.umask(027)
        sys.stdout.flush()
        sys.stdout.flush()
        devnull_fd = os.open('/dev/null', os.O_RDWR)
        os.dup2(devnull_fd, 0)
        os.dup2(devnull_fd, 1)
        os.dup2(devnull_fd, 2)
        atexit.register(lambda: os.remove(self.pidfile))
        pf = open(self.pidfile, 'w')
        pf.write('%s' % os.getpid())
        pf.close()
        self.run()

    def run(self):
        raise NotImplementedError('write your logic here')


if __name__ == '__main__':
    class DaemonExample(Daemon):
        def run(self):
            import time
            for i in range(4):
                time.sleep(5)
    daemon = DaemonExample('/tmp/daemon_example.pid')
    daemon.start()