How does one expose methods and properties on DBus using a QDBusAbstractAdaptor with PyQt4?

Question

I am trying to get some basic code running on DBus using PyQt4, specifically QtDBus. I am using a Python3 version of PyQt4. I have already gotten the code I want running on Qt (c++) but I want to get similar code running using only Python. I want to expose Methods, signals/slots and properties on DBus for other Python code to call.

In Qt you use the Q_CLASSINFO macro/function to do DBus introspection. While I have pulled in the Q_CLASSINFO method, i can't get it to produce the same type of functionality. As far as I can tell there is zero documentation on the Q_CLASSINFO method, so I'm not sure if there is another way. Using D-Feet i can clearly see that no methods are exposed automatically, so I'm kind of stuck.

Here is what I have so far.

from PyQt4 import QtDBus
from PyQt4.QtCore import QCoreApplication, QObject, Q_CLASSINFO, pyqtSlot, pyqtProperty
from PyQt4.QtDBus import QDBusConnection, QDBusAbstractAdaptor

SERVICE = 'com.home.dbus'

class MyServer(QObject):


    def __init__(self):
        QObject.__init__(self)
        self.__dbusAdaptor = ServerAdaptor(self)

    def close(self):
        pass

    def echo(self, value):
        echoed = 'Received {0}'.format(value)
        return echoed

    def name(self):
        return 'myname'

    def dbus_adaptor(self):
        return self.__dbusAdaptor

class ServerAdaptor(QDBusAbstractAdaptor):
    """ This provides the DBus adaptor to the outside world"""

    def __init__(self, parent):
        super().__init__(parent)
        self.__parent = parent
        Q_CLASSINFO("D-Bus Introspection",
        "  <interface name=\"com.home.dbus\">\n"
        "    <method name=\"name\">\n"
        "      <arg direction=\"out\" type=\"s\" name=\"name\"/>\n"
        "    </method>\n"
        "    <method name=\"echo\">\n"
        "      <arg direction=\"in\" type=\"s\" name=\"phrase\"/>\n"
        "      <arg directory=\"out\" type=\"s\" name=\"echoed\"/>\n"
        "    </method>\n"
        "  </interface>\n")

    def close(self):
        parent.close()

    def echo(self, value):
        return parent.echo(value)

    def name(self):
        return parent.name

def start():
    app = QCoreApplication([])
    if QDBusConnection.sessionBus().isConnected() == False:
        print('Cannot connect to D-Bus session bus')
        return
    print('Starting')
    server = MyServer()
    if not QDBusConnection.sessionBus().registerService(SERVICE):
        print('Unable to register service name')
        return
    if not QDBusConnection.sessionBus().registerObject('/mydbus', server.dbus_adaptor):
        print('Unable to register object at service path')
        return
    app.exec();
    print('Exited')

if __name__ == '__main__':
    start()

While I really like the using QtDBus in C++ because of how I want to structure this large project of mine, I really need the object accessed via DBus to be written in Python3.

Answer 1

There are several problems with your program. I recommend having a look at the remotecontrolledcar and pingpong examples from the latest PyQt sources, they're pretty helpful. The main points to note are:

• You should pass a MyServer instance (not ServerAdaptor) to registerObject()
• Add pyqtSlot() decorators to the functions you wish to expose via D-Bus
• Call Q_CLASSINFO() at the top of the adaptor class, not in its __init__() function
• Also set the "D-Bus Interface" using Q_CLASSINFO()
• Your introspection XML contained a typo ('directory' instead of 'direction')

Here's a stripped-down example that works for me (Python 3.2.3/Qt 4.8.2/PyQt 4.9.4):

from PyQt4 import QtDBus
from PyQt4.QtCore import (QCoreApplication, QObject, Q_CLASSINFO, pyqtSlot,
                          pyqtProperty)
from PyQt4.QtDBus import QDBusConnection, QDBusAbstractAdaptor

class MyServer(QObject):

    def __init__(self):
        QObject.__init__(self)
        self.__dbusAdaptor = ServerAdaptor(self)
        self.__name = 'myname'

    def echo(self, value):
        return'Received: {0}'.format(value)

    @property
    def name(self):
        return self.__name

    @name.setter
    def name(self, value):
        self.__name = value


class ServerAdaptor(QDBusAbstractAdaptor):
    """ This provides the DBus adaptor to the outside world"""

    Q_CLASSINFO("D-Bus Interface", "com.home.dbus")
    Q_CLASSINFO("D-Bus Introspection",
    '  <interface name="com.home.dbus">\n'
    '    <property name="name" type="s" access="readwrite"/>\n'
    '    <method name="echo">\n'
    '      <arg direction="in" type="s" name="phrase"/>\n'
    '      <arg direction="out" type="s" name="echoed"/>\n'
    '    </method>\n'
    '  </interface>\n')

    def __init__(self, parent):
        super().__init__(parent)

    @pyqtSlot(str, result=str)
    def echo(self, phrase):
        return self.parent().echo(phrase)

    @pyqtProperty(str)
    def name(self):
        return self.parent().name

    @name.setter
    def name(self, value):
        self.parent().name = value

def start():
    app = QCoreApplication([])
    bus = QDBusConnection.sessionBus()
    server = MyServer()
    bus.registerObject('/mydbus', server)
    bus.registerService('com.home.dbus')
    app.exec()

if __name__ == '__main__':
    start()