installEventFilter not working on PySide6

EddieC

I'm trying a minimal example on eventFilter without success. Here's my code:

import sys
from pathlib import Path
from PySide6.QtCore import QObject
from PySide6.QtQml import QQmlApplicationEngine
from PySide6.QtWidgets import QApplication

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

    engine = QQmlApplicationEngine()
    qml_path = Path(__file__)
    qml_file = qml_path.with_suffix('.qml')
    engine.load(qml_file)
    root_objects = engine.rootObjects()
    if not root_objects:
        sys.exit(-1)

    mainwindow = root_objects[0]

    class EventInspector(QObject):
        def eventFilter(self, obj, event):
            print('obj >>', obj)
            print('event >>', event)
            return QObject.eventFilter(obj, event)

    app.installEventFilter(EventInspector())
    sys.exit(app.exec())

When I run this code on a QML window with a couple of controls nothing happens, not a single event is printed.

I have tried to change line 26 for...

    mainwindow.installEventFilter(EventInspector())

...but nothing happens either. No event is captured by the filter in this case either. Am I misunderstanding how event filters are supposed to work in PySide6?

JonB

@EddieC
I believe you are doing it correctly, but I know nothing about QML nor PySide6.

I am a touch concerned about the lifetime of your EventInspector() as a parameter only. Will Python release this after the call? Try:

eventInspector = EventInspector()
app.installEventFilter(eventInspector)

? Otherwise what about knocking up a 4-line widgets application and trying it there, in case it's a QML issue?

EddieC

@JonB

Bingo! That solves the problem, thank you!

I am somewhat surprised that the internal reference from app to the eventInspector object is not enough to keep the object alive (as per the usual rules of Python's garbage collection). Either I missed the memo on references from Qt to Python being weakrefs, or this might be a bug on the API.

JonB

@EddieC
Yes, I too am unsure why it does not keep a reference around, but do not know the rules/implementation.

A little tip worth knowing for this kind of potential problem. There is a QObject::destroyed() signal on every QObject-derived object. If you slotted onto that from your EventInspector you would/should see it getting destroyed, and that would tell you that you needed to keep your own reference.