SImple (but specific) listener and sender Python 3 DBus example

Question

I want to make a program that has two parts. A listener (a server, if you will) and a sender (the client). I did some research and learned that this is done via a method program

Answer 1

There's not a lot of documentation for dbus in python3, but I managed to figure it out so I'll document it here: The major difference from all the python2 examples is replacing import gobject with import gi.repository.GLib.

You can find more examples (which use more features than I needed) in the dbus-python examples directory.

I didn't implement self-backgrounding in the server because that style of daemon has gone out of style recently.

common.py:

# well-known name for our program
ECHO_BUS_NAME = 'com.stackoverflow.question_21793826.EchoService'

# interfaces implemented by some objects in our program
ECHO_INTERFACE = 'com.stackoverflow.question_21793826.EchoInterface'
QUIT_INTERFACE = 'com.stackoverflow.question_21793826.QuitInterface'

# paths to some objects in our program
ECHO_OBJECT_PATH = '/EchoServerObject'

server.py:

#!/usr/bin/env python3

# standard includes
import sys

# dbus includes
import gi.repository.GLib
import dbus
import dbus.service
import dbus.mainloop.glib

# project includes
import common


class EchoServerObject(dbus.service.Object):

    # TODO it would be nice to make a better decorator using annotations:
    #   def foo(self, a: 's', b: 's') -> '': pass
    # but the existing dbus decorator does its own reflection which
    # fails if there are any annotations (or keyword-only arguments)
    @dbus.service.method(common.ECHO_INTERFACE,
            in_signature='s', out_signature='')
    def echo(self, message):
        message = str(message) # get rid of subclass for repr
        print('server: a client said %r' % message)

    @dbus.service.method(common.QUIT_INTERFACE,
            in_signature='', out_signature='')
    def quit(self):
        # this should be a separate object, but I'm
        # showing how one object can have multiple interfaces
        self.mainloop.quit()

def stop():
    bus = dbus.SessionBus()

    proxy = bus.get_object(common.ECHO_BUS_NAME, common.ECHO_OBJECT_PATH)
    iface = dbus.Interface(proxy, common.QUIT_INTERFACE)

    iface.quit()

def server():
    dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)

    bus = dbus.SessionBus()
    try:
        name = dbus.service.BusName(common.ECHO_BUS_NAME, bus, do_not_queue=True)
    except dbus.NameExistsException:
        sys.exit('Server is already running.')
    else:
        print('Server is not running yet. Putting on listening ears.')
    echo = EchoServerObject(bus, common.ECHO_OBJECT_PATH)

    mainloop = gi.repository.GLib.MainLoop()
    echo.mainloop = mainloop
    mainloop.run()

def main(exe, args):
    if args == ['stop']:
        stop()
    elif not args:
        server()
    else:
        sys.exit('Usage: %s [stop]' % exe)

if __name__ == '__main__':
    main(sys.argv[0], sys.argv[1:])

client.py:

#!/usr/bin/env python3

# standard includes
import sys

# dbus includes
import dbus

# project includes
import common


def client(mes):
    bus = dbus.SessionBus()

    try:
        proxy = bus.get_object(common.ECHO_BUS_NAME, common.ECHO_OBJECT_PATH)
    except dbus.DBusException as e:
        # There are actually two exceptions thrown:
        # 1: org.freedesktop.DBus.Error.NameHasNoOwner
        #   (when the name is not registered by any running process)
        # 2: org.freedesktop.DBus.Error.ServiceUnknown
        #   (during auto-activation since there is no .service file)
        # TODO figure out how to suppress the activation attempt
        # also, there *has* to be a better way of managing exceptions
        if e._dbus_error_name != 'org.freedesktop.DBus.Error.ServiceUnknown':
            raise
        if e.__context__._dbus_error_name != 'org.freedesktop.DBus.Error.NameHasNoOwner':
            raise
        print('client: No one can hear me!!')
    else:
        iface = dbus.Interface(proxy, common.ECHO_INTERFACE)
        iface.echo(mes)

def main(exe, args):
    if args:
        client(' '.join(args))
    else:
        sys.exit('Usage: %s message...' % exe)

if __name__ == '__main__':
    main(sys.argv[0], sys.argv[1:])