PySide6 D-Bus Signal Interface Not Emitting or Discoverable

tolland

I'm building a PySide6 desktop app that uses QDBusConnection to expose a D-Bus interface with both methods and signals.
✅ Methods work:

Registering an object with QDBusConnection.registerObject(...) allows D-Bus method calls (e.g. ping()) to show up and function correctly.

They appear in tools like qdbusviewer and respond as expected.

❌ But signals fail in two distinct scenarios:

Case 1: Registering a plain QObject with ExportAllSignals

I register it using:

conn.registerObject("/org/example", obj, QDBusConnection.ExportAllSlots | QDBusConnection.ExportAllSignals)

The signal does appear in dbus-monitor, but:

It uses a fallback interface name like local.myapp.module.MyObject (i.e. from Python module).
It has no bus name set in the dbus-monitor output.
It’s not filterable by interface name (e.g. interface='org.example.MyInterface') — so subscribers don’t catch it unless they match on path only.

Attempts to override __module__ and __qualname__ only change the string in the signal, but do not fix this interface mismatch.

I attach case1 which reproduces that problem

import sys
from PySide6.QtCore import QObject, Signal, QTimer
from PySide6.QtWidgets import QApplication
from PySide6.QtDBus import QDBusConnection
import signal

signal.signal(signal.SIGINT, signal.SIG_DFL)


class MyObject(QObject):
    my_signal = Signal(str)

    def __init__(self):
        super().__init__()
        self.timer = QTimer(self)
        self.timer.timeout.connect(self.send_signal)
        self.timer.start(1000)

    def send_signal(self):
        print("Emitting signal...")
        self.my_signal.emit("hello from D-Bus")


if __name__ == "__main__":
    app = QApplication(sys.argv)
    obj = MyObject()

    conn = QDBusConnection.sessionBus()
    if not conn.registerService("org.example.TestApp"):
        print("❌ Failed to register service")
        sys.exit(1)

    flags = QDBusConnection.RegisterOption.ExportAllSignals
    if not conn.registerObject(
        "/org/example/TestObject",
        "org.example.interface",
        obj,
        flags,
    ):
        print("❌ Failed to register object")
        sys.exit(1)

    print("✅ Service and object registered. Use `dbus-monitor` to observe.")
    sys.exit(app.exec())

running it like so:

$ python scripts/case1.py
✅ Service and object registered. Use `dbus-monitor` to observe.
Emitting signal...
Emitting signal...
Emitting signal...
Emitting signal...

we see the signal through introspection:

[~] $ qdbus -h org.example.TestApp /org/example/TestObject
signal void org.example.interface.my_signal(QString)
method QDBusVariant org.freedesktop.DBus.Properties.Get(QString interface_name, QString property_name)
method QVariantMap org.freedesktop.DBus.Properties.GetAll(QString interface_name)
signal void org.freedesktop.DBus.Properties.PropertiesChanged(QString interface_name, QVariantMap changed_properties, QStringList invalidated_properties)
method void org.freedesktop.DBus.Properties.Set(QString interface_name, QString property_name, QDBusVariant value)
method QString org.freedesktop.DBus.Introspectable.Introspect()
method QString org.freedesktop.DBus.Peer.GetMachineId()
method void org.freedesktop.DBus.Peer.Ping()

signal shows as follows:

C
[~] $ dbus-monitor "type='signal'" --profile | grep example
sig     1742977053.198865       16      :1.1200 <none>  /org/example/TestObject local.py.case1.MyObject        my_signal
sig     1742977054.148935       17      :1.1200 <none>  /org/example/TestObject local.py.case1.MyObject        my_signal
sig     1742977055.111065       18      :1.1200 <none>  /org/example/TestObject local.py.case1.MyObject        my_signal

It’s not filterable by interface name (e.g. interface='org.example.MyInterface') — so subscribers don’t catch it unless they match on path only.

dbus-next won't allow attaching to the empty dbus service name. So its not that useful, though i guess some exhaustive searching all services and paths for interface.

Case 2: Using QDBusAbstractAdaptor

I create an adaptor class subclassing QDBusAbstractAdaptor, define Signal attributes and @Slot methods.

case2.py

# case2_adaptor_signal_fail.py
import sys
import signal
from PySide6.QtCore import QObject, Signal, Slot, QTimer
from PySide6.QtWidgets import QApplication
from PySide6.QtDBus import QDBusConnection, QDBusAbstractAdaptor

signal.signal(signal.SIGINT, signal.SIG_DFL)

SERVICE_NAME = "org.example.TestApp"
OBJECT_PATH = "/org/example/TestObject"
INTERFACE_NAME = "org.example.interface"


class BackendObject(QObject):
    def __init__(self):
        super().__init__()
        self.setObjectName("BackendObject")


class MyAdaptor(QDBusAbstractAdaptor):
    appLaunched = Signal(str)

    def __init__(self, parent: QObject):
        super().__init__(parent)
        self.setAutoRelaySignals(True)
        self._timer = QTimer(self)
        self._timer.timeout.connect(self.emit_signal)
        self._timer.start(1000)

    @Slot()
    def emit_signal(self):
        print("🚨 Emitting appLaunched...")
        self.appLaunched.emit("hello from QDBusAbstractAdaptor")


if __name__ == "__main__":
    app = QApplication(sys.argv)

    backend = BackendObject()
    adaptor = MyAdaptor(backend)

    conn = QDBusConnection.sessionBus()
    if not conn.registerService(SERVICE_NAME):
        print("❌ Failed to register service")
        sys.exit(1)

    flags = QDBusConnection.RegisterOption.ExportAllSignals
    if not conn.registerObject(
        OBJECT_PATH,
        "org.example.interface",
        adaptor,
        flags,
    ):
        print("❌ Failed to register object")
        sys.exit(1)

    print(f"✅ Running at {SERVICE_NAME} {OBJECT_PATH}")
    sys.exit(app.exec())

I register the adaptor on a parent QObject and register the parent using:

conn.registerObject("/org/example", interface, parent, QDBusConnection.ExportAdaptors)

In this case:

Introspection works — signals and methods appear in qdbusviewer and under the correct org.example.MyInterface name.

[~] $ qdbus -h org.example.TestApp /org/example/TestObject
signal void org.example.interface.appLaunched(QString)
method QDBusVariant org.freedesktop.DBus.Properties.Get(QString interface_name, QString property_name)
method QVariantMap org.freedesktop.DBus.Properties.GetAll(QString interface_name)
signal void org.freedesktop.DBus.Properties.PropertiesChanged(QString interface_name, QVariantMap changed_properties, QStringList invalidated_properties)
method void org.freedesktop.DBus.Properties.Set(QString interface_name, QString property_name, QDBusVariant value)
method QString org.freedesktop.DBus.Introspectable.Introspect()
method QString org.freedesktop.DBus.Peer.GetMachineId()
method void org.freedesktop.DBus.Peer.Ping()

But when I emit the signal from the adaptor using self.appLaunched.emit("Hello"), nothing appears on the D-Bus (not even in dbus-monitor).

$ python scripts/case2.py
✅ Running at org.example.TestApp /org/example/TestObject
🚨 Emitting appLaunched...
🚨 Emitting appLaunched...
🚨 Emitting appLaunched...
🚨 Emitting appLaunched...
🚨 Emitting appLaunched...
🚨 Emitting appLaunched...

nothing on the dbus-monitor

[~] $ dbus-monitor "type='signal'" --profile | grep example

Things tried so far:

[OK] QObject registered directly -> Signal appears on bus, but messy interface name (e.g. local.module.ClassName) and no bus_name
[FAIL] QDBusAbstractAdaptor used -> Interface visible in qdbusviewer, but signals never appear on dbus-monitor
[FAIL] Adaptor registered directly -> Nothing works; introspection and signals both fail
[FAIL] adaptor.emit() on adaptor -> No effect on D-Bus (signal doesn’t show up)
[FAIL] setAutoRelaySignals(True) used -> No help; signal still doesn’t emit on D-Bus
[FAIL] setting __classinfo__ = [("D-Bus Interface", "org.example.interface")] on adaptor and QObject
[FAIL] setting BackendObject.__module__ = "org.example.interface"
[FAIL] setting BackendObject.__qualname__ = "Interface"

SGaist

Hi and welcome to devnet,

Thanks for the detailed post and sorry I can't help you directly.
Things that would help further:

which version of PySide6 are you using ?
which Linux distribution are you running ?
which version of DBus do you have ?