how to ListUnits of systemd user units via DBus?

[dckc]

I'm trying to use systemd to manage a node.js service (finquick @@IOU link), and
I'd like something on my taskbar to tell me whether it's running. But I can't
figure out how to get the ListUnits dbus method to work on the SessionBus.

Works as expected:

$ dbus-send --system --print-reply --reply-timeout=2000 --type=method_call   --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager.ListUnits
...
         object path "/org/freedesktop/systemd1/unit/devfsd_2eservice"
         uint32 0
         string ""
         object path "/"
      }
   ]

fails:

connolly@jambox:~$ dbus-send --print-reply --reply-timeout=2000 --type=method_call   --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager.ListUnits
Error org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.freedesktop.systemd1 exited with status 1

I started out using python Popen and systemctl --user status --all --output=json but --output=json doesn't apply to status of units; only to journal entries. In a Jun 2015 discussion, the systemd folks endorse DBus for this sort of thing.

https://zignar.net/2014/09/08/getting-started-with-dbus-python-systemd/ works just fine, but only for the SystemBus. When I try the SessionBus, I lose:

python my_services_applet.py
Traceback (most recent call last):
  File "my_services_applet.py", line 123, in <module>
    _test1()
  File "my_services_applet.py", line 55, in _test1
    manager = systemd_manager(SessionBus())
  File "my_services_applet.py", line 42, in systemd_manager
    '/org/freedesktop/systemd1')
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 241, in get_object
    follow_name_owner_changes=follow_name_owner_changes)
...
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.freedesktop.systemd1 exited with status 1

systemd/systemd#5247 close, but arch (and Fedora?). I'm using Ubuntu.