DBus Support

PyQt5 provides two different modules that implement support for DBus. The QtDBus module provides wrappers for the standard Qt DBus classes. The dbus.mainloop.pyqt5 module adds 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.pyqt5

The dbus.mainloop.pyqt5 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.pyqt5 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 PyQt5:

from dbus.mainloop.pyqt5 import DBusQtMainLoop

DBusQtMainLoop(set_as_default=True)