Dbus Tutorial - Fun with Network Manager

Introduction
Introspection: figuring out the rules 
Fun with Network Manager
Create a service 
Gobject Introspection


Let's figure out how to use  dbus to get detailed information out of Network Manager (NM), and then to plug our own information back into NM.

Example #1: This script determines if a specific network is the active connection. This is handy for, say, mapping network printers or networked drives, or setting the time, or automating backups, or lots of other stuff.

#!/bin/sh
# What network am I on?

# Get the Active Connection path
# Result should be like: /org/freedesktop/NetworkManager/ActiveConnection/187
active_connection_path=$( dbus-send --system --print-reply \
                          --dest=org.freedesktop.NetworkManager \
                          /org/freedesktop/NetworkManager \
                          org.freedesktop.DBus.Properties.Get \
                          string:"org.freedesktop.NetworkManager" \
                          string:"ActiveConnections" \
                          | grep ActiveConnection/ | cut -d'"' -f2 )

# Get the Access Point path
# Result should be like: /org/freedesktop/NetworkManager/AccessPoint/194
access_point_path=$( dbus-send --system --print-reply \
                     --dest=org.freedesktop.NetworkManager \
                     "$active_connection_path" \
                     org.freedesktop.DBus.Properties.Get \
                     string:"org.freedesktop.NetworkManager.Connection.Active" \
                     string:"SpecificObject" \
                     | grep variant | cut -d'"' -f2 )

# Get the Access Point ESSID
# Result should be something like "NETGEAR"
essid=$( dbus-send --system --print-reply \
                   --dest=org.freedesktop.NetworkManager \
                   "$access_point_path" \
                   org.freedesktop.DBus.Properties.Get \
                   string:"org.freedesktop.NetworkManager.AccessPoint" \
                   string:"Ssid" \
                   | grep variant | cut -d'"' -f2 )



# If we are on the HOME network
if [ "$essid"=="MyHomeNet" ]; then
   # Do network-specific changes here

elif [ "$essid"=="WorkCorporateNet" ]
   # Do network-specific changes here

else
   # Do changes for unrecognized network or no network at all here

fi
exit 0

Example #2: Disable networking

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager \
            org.freedesktop.DBus.Properties.Set \
            string:"org.freedesktop.NetworkManager" \
            string:"NetworkingEnabled" \
            variant:boolean:false

Example #3: Enable networking. It's exactly the same as the previous example, except for the last line.

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager \
            org.freedesktop.DBus.Properties.Set \
            string:"org.freedesktop.NetworkManager" \
            string:"NetworkingEnabled" \
            variant:boolean:true

Example #4: Check networking status

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager" \
            string:"NetworkingEnabled"

method return sender=:1.4 -> dest=:1.325 reply_serial=2
   variant       boolean true

Example #5: Disable Wireless

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager \
            org.freedesktop.DBus.Properties.Set \
            string:"org.freedesktop.NetworkManager" \
            string:"WirelessEnabled" \
            variant:boolean:false

Example #6: Enable Wireless

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager \
            org.freedesktop.DBus.Properties.Set \
            string:"org.freedesktop.NetworkManager" \
            string:"WirelessEnabled" \
            variant:boolean:true

Example #7: Check Wireless Status

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager" \
            string:"WirelessEnabled"

method return sender=:1.4 -> dest=:1.326 reply_serial=2
   variant       boolean true

Example #8:

Example #9: List all active network connections

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager" \
            string:"ActiveConnections"

method return sender=:1.4 -> dest=:1.328 reply_serial=2
   variant       array [
         object path "/org/freedesktop/NetworkManager/ActiveConnection/18"
      ]

Example #10: Which interface is the active connection using?

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager/ActiveConnection/18 \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager.Connection.Active" \
            string:"Devices"

method return sender=:1.4 -> dest=:1.331 reply_serial=2
   variant       array [
         object path "/org/freedesktop/NetworkManager/Devices/0"

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager/Devices/0 \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager.Device" \
            string:"Interface"

method return sender=:1.4 -> dest=:1.332 reply_serial=2
   variant       string "wlan0"

Example #11: Get the current wireless access point

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager/ActiveConnection/18 \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager.Connection.Active" \
            string:"SpecificObject"

method return sender=:1.4 -> dest=:1.334 reply_serial=2
   variant       object path "/org/freedesktop/NetworkManager/AccessPoint/209"

Example #12: Get the list of all visible wireless access points

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager 
            /org/freedesktop/NetworkManager/Devices/0 \
            org.freedesktop.NetworkManager.Device.Wireless.GetAccessPoints

method return sender=:1.4 -> dest=:1.333 reply_serial=2
   array [
      object path "/org/freedesktop/NetworkManager/AccessPoint/209"
      object path "/org/freedesktop/NetworkManager/AccessPoint/208"
      object path "/org/freedesktop/NetworkManager/AccessPoint/207"
      object path "/org/freedesktop/NetworkManager/AccessPoint/206"
   ]

Example #13: Read the SSID of the active wireless access point

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager/AccessPoint/209 \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager.AccessPoint" \
            string:"Ssid"

method return sender=:1.4 -> dest=:1.335 reply_serial=2
   variant       array of bytes "NETGEAR"

Example #14: Read the signal strength of the active wireless point

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager/AccessPoint/209 \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager.AccessPoint" \
            string:"Strength"

method return sender=:1.4 -> dest=:1.340 reply_serial=2
   variant       byte 54
# 54 in byte (hexadecimal) = 84 in decimal. Strength = 84%

Example #15: Read the stored NM connection that is currently active

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager/ActiveConnection/18 \
            org.freedesktop.DBus.Properties.Get \
            string:"org.freedesktop.NetworkManager.Connection.Active" \
            string:"Connection"

method return sender=:1.4 -> dest=:1.379 reply_serial=2
   variant       object path "/org/freedesktop/NetworkManager/Settings/10"

Example #16: Disconnect from the current network connection (auto-reconnect)

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager \
            org.freedesktop.NetworkManager.DeactivateConnection \
            objpath:"/org/freedesktop/NetworkManager/ActiveConnection/18"

method return sender=:1.4 -> dest=:1.370 reply_serial=2

Example #17: Disconnect from the current network connection and stay disconnected

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager/Devices/0 \
            org.freedesktop.NetworkManager.Device.Disconnect

method return sender=:1.4 -> dest=:1.354 reply_serial=2

Example #18: Connect to a specific wireless network

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            org/freedesktop/NetworkManager \
            org.freedesktop.NetworkManager.ActivateConnection \
            objpath:"/org/freedesktop/NetworkManager/Settings/10" \
            objpath:"/org/freedesktop/NetworkManager/Devices/0" \
            objpath:"/org/freedesktop/NetworkManager/AccessPoint/209"

method return sender=:1.4 -> dest=:1.382 reply_serial=2
   object path "/org/freedesktop/NetworkManager/ActiveConnection/20"

Example #19: Dump all information about an access point

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            /org/freedesktop/NetworkManager/Settings/10 \
            org.freedesktop.NetworkManager.Settings.Connection.GetSettings

method return sender=:1.4 -> dest=:1.386 reply_serial=2
   array [## EDITED - it's really long]

Dbus Tutorial - Introspection: Figuring Out The Rules

Introduction
Introspection
Network Manager 
Create a Service
Gobject Introspection


Last time, we discussed what to use dbus for and the basics of structuring a dbus command. We went over how to structure the grammar of a command so it makes sense (mapping the destination, path, method, and message elements), and we went over the syntax (stringing together those elements in a coherent way).

This lesson is about figuring out what methods are available and how to use them.


Introspection

dbus is introspectable. That means you can ask dbus what commands are available.

Here's an introspection example. You can see that the return is XML wrapped inside a string (you don't need to read it all):

$ dbus-send --system --print-reply --dest=org.freedesktop.NetworkManager /org/freedesktop/NetworkManager org.freedesktop.DBus.Introspectable.Introspect
method return sender=:1.4 -> dest=:1.441 reply_serial=2
   string "

<node>
  <interface name="org.freedesktop.DBus.Introspectable">
    <method name="Introspect">
      <arg direction="out" name="data" type="s">
    </arg></method>
  <interface name="org.freedesktop.DBus.Properties">
    <method name="Get">
      <arg direction="in" name="interface" type="s">
      <arg direction="in" name="propname" type="s">
      <arg direction="out" name="value" type="v">
    </arg></arg></arg></method>
    <method name="Set">
      <arg direction="in" name="interface" type="s">
      <arg direction="in" name="propname" type="s">
      <arg direction="in" name="value" type="v">
    </arg></arg></arg></method>
    <method name="GetAll">
      <arg direction="in" name="interface" type="s">
      <arg direction="out" name="props" type="a{sv}">
    </arg></arg></method>
  </interface>
  <interface name="org.freedesktop.NetworkManager">
    <method name="state">
      <arg direction="out" name="state" type="u">
    </arg></method>
    <method name="SetLogging">
      <arg direction="in" name="level" type="s">
      <arg direction="in" name="domains" type="s">
    </arg></arg></method>
    <method name="GetPermissions">
      <arg direction="out" name="permissions" type="a{ss}">
    </arg></method>
    <method name="Enable">
      <arg direction="in" name="enable" type="b">
    </arg></method>
    <method name="Sleep">
      <arg direction="in" name="sleep" type="b">
    </arg></method>
    <method name="DeactivateConnection">
      <arg direction="in" name="active_connection" type="o">
    </arg></method>
    <method name="AddAndActivateConnection">
      <arg direction="in" name="connection" type="a{sa{sv}}">
      <arg direction="in" name="device" type="o">
      <arg direction="in" name="specific_object" type="o">
      <arg direction="out" name="path" type="o">
      <arg direction="out" name="active_connection" type="o">
    </arg></arg></arg></arg></arg></method>
    <method name="ActivateConnection">
      <arg direction="in" name="connection" type="o">
      <arg direction="in" name="device" type="o">
      <arg direction="in" name="specific_object" type="o">
      <arg direction="out" name="active_connection" type="o">
    </arg></arg></arg></arg></method>
    <method name="GetDeviceByIpIface">
      <arg direction="in" name="iface" type="s">
      <arg direction="out" name="device" type="o">
    </arg></arg><<method>
    <method name="GetDevices">
      <arg direction="out" name="devices" type="ao">
    </arg></method>
     <signal name="DeviceRemoved">
       <arg type="o">
     </arg> </signal>
     <signal name="DeviceAdded">
       <arg type="o">
     </arg> </signal>
     <signal name="PropertiesChanged">
       <arg type="a{sv}">
     </arg> </signal>
     <signal name="StateChanged">
       <arg type="u">
     </arg> </signal>
     <signal name="CheckPermissions">
     </signal>
     <property access="read" name="State" type="u">
     <property access="read" name="Version" type="s">
     <property access="read" name="ActiveConnections" type="ao">
     <property access="read" name="WimaxHardwareEnabled" type="b">
     <property access="readwrite" name="WimaxEnabled" type="b">
     <property access="read" name="WwanHardwareEnabled" type="b">
     <property access="readwrite" name="WwanEnabled" type="b">
     <property access="read" name="WirelessHardwareEnabled" type="b">
     <property access="readwrite" name="WirelessEnabled" type="b">
     <property access="read" name="NetworkingEnabled" type="b">
   </property> </property> </property> </property> </property> </property> </property> </property> </property> </property>
   <node name="AccessPoint">
   <node name="ActiveConnection">
   <node name="AgentManager">
   <node name="DHCP4Config">
   <node name="Devices">
   <node name="IP4Config">
   <node name="Settings">
 </node>
" </node> </node> </node> </node> </node> </node>

Let's see if we can translate this into something more human readable.

For example,

<node>
  <interface name="org.freedesktop.DBus.Introspectable">
    <method name="Introspect">
      <arg direction="out" name="data" type="s">
    </arg&gt;&lt;/method> 

  ... 

</node>

is the method we used for introspection (remember?).

See the org.freedesktop.DBus.Introspectable.Introspect?  The arg direction "out" means the the response, and type "d" means data string).


How to use introspection to create a dbus query or command

Here's a slightly more complex example of an introspection response:

  <interface name="org.freedesktop.DBus.Properties">
    ...
    <method name="GetAll">
      <arg direction="in" name="interface" type="s">
      <arg direction="out" name="props" type="a{sv}">
    </arg> </arg> </method>

    ...

  <interface name="org.freedesktop.NetworkManager"> 

This generic method, org.freedesktop.DBus.Properties.GetAll, returns a complete dump of all of some other interface's properties.

Let's go back to grammar (destination, path, method, message) and say that again:

I want to see a dump of Network manager's top-level properties.
"Hey Network Manager, please give me a printout of all of Network Manager's top-level properties."

Destination: "Hey, Network Manager": org.freedesktop.NetworkManager

Path:  "Network Manager's": org/freedesktop/NetworkManager

Method: "Give me a printout of properties": org.freedesktop.DBus.Properties.GetAll

Message: "top-level": org.freedesktop.NetworkManager

You probably noticed that this example has some duplication. When working with dbus, get used to it.


Now let's put it into the right syntax:

dbus-send   [ --system| --session] --print-reply --dest=DEST PATH METHOD [MESSAGE]

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            org/freedesktop/NetworkManager \
            org.freedesktop.DBus.Properties.GetAll \
            string:"org.freedesktop.NetworkManager"

Let's do one more slightly different example:

<interface name="org.freedesktop.NetworkManager">
    ...
    <method name="ActivateConnection">
      <arg direction="in" name="connection" type="o">
      <arg direction="in" name="device" type="o">
      <arg direction="in" name="specific_object" type="o">
      <arg direction="out" name="active_connection" type="o">
    </arg> </arg> </arg> </arg> </method>

 </node>
" </node> </node> </node> </node> </node> </node>

This method, ActivateConnection, makes a known connection into the active network connection. For example, when switching from one access point to another. There are three extra pieces of information needed, and network manager returns one piece of information in the response.

Let's go back to grammar (destination, path, method, message) and say that again:

"Hey Network Manager, make Access Point Foo (using wireless device 0 and network password settings 67) the active connection."

Destination: "Hey, Network Manager": org.freedesktop.NetworkManager

Path:  "Network Manager": org/freedesktop/NetworkManager

Method: "Make...the active connection": org.freedesktop.NetworkManager.ActivateConnection

Message 1: "Access Point Foo": org/freedesktop/NetworkManager/AccessPoint/220

Message 2: "wireless device 0": org/freedesktop/NetworkManager/Device/0

Message 3: "network password info 67": org/freedesktop/NetworkManager/Settings/67

Now let's put it into the right syntax:

dbus-send   [ --system| --session] --print-reply --dest=DEST PATH METHOD [MESSAGE]

$ dbus-send --system --print-reply \
            --dest=org.freedesktop.NetworkManager \
            org/freedesktop/NetworkManager \
            org.freedesktop.NetworkManager/ActivateConnection \
            objpath:"org/freedesktop/NetworkManager/AccessPoint/220" \

            objpath:"org/freedesktop/NetworkManager/Device/0" \
            objpath:"org/freedesktop/NetworkManager/Settings/67"

Obviously, this example won't work for you unless you do the introspection to find valid Access Points, Devices, and Settings that work together. dbus will tell you a lot of it...if you ask...but it's not a user-friendly graphical user interface. You need to ask the right questions and use your own logic.

Making introspection easier is where d-feet comes in.


d-feet makes introspection easy

d-feet is a python application (part of the d-feet package in Debian and Ubuntu) that does introspection for you while you write your program.

In this screenshot, you can see lots of the same introspection information that we retrieved before. Easier to read and understand, isn't it? See how the Interfaces and Methods are listed for each Object Path?

Without d-feet, use the following dbus-send command to find out what's available on the bus:

$ dbus-send --session --print-reply --dest="org.freedesktop.DBus" /org/freedesktop/DBus org.freedesktop.DBus.ListActivatableNames

Now you know how to find the information you need to use dbus properly.

Using DBus on Xubuntu 8.04

This post is obsolete has been superseded by my 2012 dbus tutorial series.

DBus is a system that permits different applications to exchange information. Tutorial Reference Other Reference.
Sometimes, DBus crashes upon restart from a suspend or hibernation. These bash commands will help you figure out if it has crashed, and how to restart it.

$ps -e | grep `cat /var/run/dbus/pid` # Confirm if DBus is running by checking for the PID number in the list of live processes.
                                      # If DBus is running, this will return the process number. 
                                      # If not, it will return nothing.
$sudo rm /var/run/dbus/pid            # Remove the stale pid file so DBus can be restarted.
$sudo dbus-daemon                     # Start DBus again.

A python script uses DBus to see if the network connection is available by asking Network Manager:

#! /usr/bin/env python
import dbus
bus = dbus.SystemBus()
item = 'org.freedesktop.NetworkManager'
eth0_path = '/org/freedesktop/NetworkManager/Devices/eth0'
eth1_path = '/org/freedesktop/NetworkManager/Devices/eth1'
interface = 'org.freedesktop.NetworkManager.Devices'

# There are two possible network interfaces: eth0 (wired) and eth1 (wireless).
eth0 = dbus.Interface(bus.get_object(item, eth0_path), interface)
if eth0.getLinkActive(): print('The wired network is up') # getLinkActive() is a boolean, TRUE if the network link is active
eth1 = dbus.Interface(bus.get_object(item, eth1_path), interface)
if eth1.getLinkActive(): print('The wireless network is up')

This shell script does exactly the same thing, using the same DBus call:

# This shell script checks Network Manager if the network is up, using dbus as the communications medium.
# There are two possible network interfaces: eth0 (wired) and eth1 (wireless). Of course, you may need to alter these to meet your own circumstances.
# The basic format of dbus-send is: dbus-send --system --dest=org.freedesktop.NetworkManager /org/freedesktop/NetworkManager/Devices/eth0 --print-reply org.freedesktop.NetworkManager.Devices.eth0.getLinkActive
DEST='org.freedesktop.NetworkManager'
PPATH='/org/freedesktop/NetworkManager/Devices'
DEVICE='org.freedesktop.NetworkManager.Devices'

result_eth0=`dbus-send --system --dest=$DEST $PPATH'/eth0' --print-reply $DEVICE'.eth0.getLinkActive'`
shaved_eth0=`echo $result_eth0 | cut -d ' ' -f8`
if [ $shaved_eth0 = 'true' ]; then echo 'The wired network is up'; fi

result_eth1=`dbus-send --system --dest=$DEST $PPATH'/eth1' --print-reply $DEVICE'.eth1.getLinkActive'`
shaved_eth1=`echo $result_eth1 | cut -d ' ' -f8`
if [ $shaved_eth1 = 'true' ]; then echo 'The wireless network is up'; fi

A Python script that queries Network Manager to get the list of wireless networks.

import dbus
item = 'org.freedesktop.NetworkManager'
path = '/org/freedesktop/NetworkManager'
interface = item + '.Device'

network_list = []
bus = dbus.SystemBus()

# Create a Network Manager interface and get the list of network devices
event = dbus.Interface(bus.get_object(item, path), interface)

# Create an interface for each device
# Query each interface to see if it's wireless
# Query each wireless interface for the networks it sees
for device in event.getDevices():
    device_interface = dbus.Interface(bus.get_object(item, device), interface)
    if device_interface.getType() == 2:  # 0 unknown, 1 wired, 2 wireless
        network_list.extend(device_interface.getNetworks())

# Reformat the network names in the list to be more readable
if network_list:
    for entry in network_list:
        #print entry    # String of path/to/network_name
        entry_list = entry.split('/')
        print entry_list[-1]  # String of network_name

A Python listener that catches the changes in wireless signal strength using both available methods.

import dbus, gobject
from dbus.mainloop.glib import DBusGMainLoop

def print_device_strength1(*args):  #Use the received signals
    signal_strength = args[1]
    print ('Signal Strength (Method 1): ' + str(signal_strength) + '%')

def print_device_strength2(*args, **kwargs):  #Use the received signals
    signal_strength = args[1]
    print ('Signal Strength (Method 2): ' + str(signal_strength) + '%')

DBusGMainLoop(set_as_default=True)    # Set up the event loop before connecting to the bus
bus_object = dbus.SystemBus()

# The variables you need. I used the shell command 'dbus-monitor --system' to find this information
sender = 'org.freedesktop.NetworkManager'
path = '/org/freedesktop/NetworkManager'
interface = sender
member = 'DeviceStrengthChanged'

# Method 1 - bus_object.proxy_object.connect_to_signal(method, action, filter, message_parts)
proxy_object = bus_object.get_object(sender, path)
proxy_object.connect_to_signal(member, print_device_strength1, dbus_interface = interface)

# Method 2 - bus_object.add_signal_receiver(action, [filters])
bus_object.add_signal_receiver(print_device_strength2, dbus_interface = interface, member_keyword = member)

# Start the loop
loop = gobject.MainLoop()
loop.run()

Thunar responds beautifully to D-Bus. Introspection is fully set up, so it's easy to use with the d-feet application. Useful for launching programs, opening folders and windows, and manipulating the trash. Launching a program by this method means that the the window manager launches the program, not the script or terminal, so the program can remain open after the script or terminal terminates.

#!/usr/bin/env python
import dbus
item = ('org.xfce.Thunar')
path = ('/org/xfce/FileManager')
interface = ('org.xfce.FileManager')
event = dbus.Interface(dbus.SessionBus().get_object(item, path), interface)

# These three lines at the end of the script open the file's 'properties' window
display = (':0')         # The current session screen
uri = ('/home/me/dbus_test.py')
event.DisplayFileProperties(uri, display)

# These three lines at the end of the script launch a new application
display = (':0')         # The current session screen
uri = ('/usr/bin/gftp-gtk')
event.Launch(uri, display)

# These four lines at the end of the script open a folder window and optionally select a file
display = (':0')         # The current session screen
uri = ('/home/me/.cron')
filename = ('anacrontab.daily')
event.DisplayFolderAndSelect(uri, filename, display)

A sample hal script.

#!/usr/bin/python
"""This python 2.5 script uses dbus to check if the lid switch is open.
Based on an original python script at http://schurger.org/wordpress/?p=49"""
import dbus

dest = 'org.freedesktop.Hal'
hal_path = '/org/freedesktop/Hal/Manager'
hal_interface = 'org.freedesktop.Hal.Manager'
udi_interface = 'org.freedesktop.Hal.Device'

# Get the list of possible input switches. The return is a list of paths.
bus = dbus.SystemBus()
hal = dbus.Interface(bus.get_object(dest, hal_path), hal_interface)
list_of_udi_paths = hal.FindDeviceByCapability('input.switch')

# Filter the list for the word 'lid'. Print the status for each one.
for udi_path in list_of_udi_paths:
    udi = dbus.Interface(bus.get_object(dest, udi_path), udi_interface)
    if udi.GetProperty('button.type') == "lid":
        # The button.state.value is FALSE if the lid is open.
        if udi.GetProperty('button.state.value'): print ('Lid is closed')
        else: print ('Lid is open') 
    else: print ('Problem: I could not find the lid switch. Sorry.')

Notes:

• The D-feet application is very handy for exploring DBus, and figuring out how to communicate with it. It's available in the Ubuntu repositories.
• More information on the destination/path/interface settings is available from each application's XML config files, found in the /etc/dbus-1/system.d and /session.d directories.
• The system-tools-backends DBus interfaces look promising, with methods for network interfaces, time, users and groups, and more. But I couldn't get any of it to work. One hint suggested that the DBus message must be sent by root instead of user.
• xfce4-terminal has a Launch method, seemingly for launching items in the terminal (source code). I can see how that would be handy, but I couldn't get it to work.