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# DBus Support

PyQt4 provides two different modules that implement support for DBus. The `QtDBus` module provides wrappers for the standard Qt DBus classes. The `dbus.mainloop.qt` module add support for the Qt event loop to the standard `dbus-python` Python module.

## `QtDBus`

The `QtDBus` module is used in a similar way to the C++ library it wraps. The main difference is in the way it supports the demarshalling of DBus structures. C++ relies on the template-based registration of types using `qDBusRegisterMetaType()` which isn’t possible from Python. Instead a slot that accepts a DBus structure in an argument should specify a slot with a single `QDBusMessage` argument. The implementation of the slot should then extract the arguments from the message using its `arguments()` method.

For example, say we have a DBus method called `setColors()` that has a single argument that is an array of structures of three integers (red, green and blue). The DBus signature of the argument would then be `a(iii)`. In C++ you would typically define a class to hold the red, green and blue values and so your code would include the following (incomplete) fragments:

    struct Color
    {
        int red;
        int green;
        int blue;
    };
    Q_DECLARE_METATYPE(Color)

    qDBusRegisterMetaType<Color>();

    class ServerAdaptor : public QDBusAbstractAdaptor
    {
        Q_OBJECT

    public slots:
        void setColors(QList<const Color &> colors);
    };

The Python version is, of course, much simpler:

    class ServerAdaptor(QDBusAbstractAdaptor):

        @pyqtSlot(QDBusMessage)
        def setColors(self, message):
            # Get the single argument.
            colors = message.arguments()[0]

            # The argument will be a list of 3-tuples of ints.
            for red, green, blue in colors:
                print("RGB:", red, green, blue)

Note that this technique can be used for arguments of any type, it is only require if DBus structures are involved.

## `dbus.mainloop.qt`

The `dbus.mainloop.qt` module provides support for the Qt event loop to `dbus-python`. The module’s API is almost identical to that of the `dbus.mainloop.glib` modules that provides support for the GLib event loop.

The `dbus.mainloop.qt` module contains the following function.

`DBusQtMainLoop`(set_as_default=False)

  Create a `dbus.mainloop.NativeMainLoop` object that uses the the Qt event loop.

  Parameters: | set_as_default – is optionally set to make the main loop instance the default for all new Connection and Bus instances. It may only be specified as a keyword argument, and not as a positional argument.

The following code fragment is all that is normally needed to set up the standard `dbus-python` language bindings package to be used with PyQt4:

    from dbus.mainloop.qt import DBusQtMainLoop

    DBusQtMainLoop(set_as_default=True)

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