PyQt 5.6: connecting to a DBus signal hangs

Question

I'm trying to connect a slot to a signal emitted through DBus in PyQt 5.6 with Python 3.5.

When I run my script like this QDBUS_DEBUG=1 python3 qtdbustest.py it never reaches the call to print('Connected') but instead just hangs at the bus.connect(...) call. The signal is seen on the bus as evident in the debug output:

QDBusConnectionPrivate(0x7f3e60002b00) : connected successfully QDBusConnectionPrivate(0x7f3e60002b00) got message (signal): QDBusMessage(type=Signal, service="org.freedesktop.DBus", path="/org/freedesktop/DBus", interface="org.freedesktop.DBus", member="NameAcquired", signature="s", contents=(":1.137") ) QDBusConnectionPrivate(0x7f3e60002b00) delivery is suspended

Here is my minimal working example:

#!/usr/bin/python3

import sys

from PyQt5.QtCore import QObject, pyqtSlot
from PyQt5.QtWidgets import QApplication
from PyQt5.QtDBus import QDBusConnection, QDBusMessage


class DbusTest(QObject):

    def __init__(self):
        super(DbusTest, self).__init__()
        bus = QDBusConnection.systemBus()
        bus.connect(
            'org.freedesktop.DBus',
            '/org/freedesktop/DBus',
            'org.freedesktop.DBus',
            'NameAcquired',
            self.testMessage
        )
        print('Connected')

    @pyqtSlot(QDBusMessage)
    def testMessage(self, msg):
        print(msg)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    discoverer = DbusTest()
    sys.exit(app.exec_())

What am I doing wrong? There must be something I overlooked so that the call to bus.connect(...) actually returns.

ekhumoro ekhumoro · Accepted Answer · 2016-07-01T18:19:53

I was able to fix your example like this:

    bus = QDBusConnection.systemBus()
    bus.registerObject('/', self)
    bus.connect( ...

However, I have to admit I don't exactly understand why it works (which is to say, I couldn't find any corroborating documentation). It does seem to make sense that you'd need to register the receiver object before attempting to make the connection, though.

PyQt 5.6: connecting to a DBus signal hangs

1 Answers