QQmlApplicationEngine list index out of range issue - qt

Question

I am trying to run the last 2 examples from this website and end up with
root = engine.rootObjects()[0] # type: QObject IndexError: list index out of range error for both examples (Signal connection for root layout) (Signal connection of other qml file)
If it helps, I put all the example files onto the same directory level and changed the import statements to Pyside6 (from PySide6.QtCore...) instead of PySide2. I also added my version of the code if it makes it easier to see:
main.py:
import sys
from PySide6.QtCore import *
from PySide6.QtQml import QQmlApplicationEngine
from PySide6.QtWidgets import QApplication
def say(s):
print(s)
if __name__ == '__main__':
app = QApplication()
engine = QQmlApplicationEngine()
engine.load(QUrl.fromLocalFile('main.qml'))
# Get the root object.
root = engine.rootObjects()[0] # type: QObject
# Find the target object. Since our target object is Window, which is the root object. So use it directly.
target_view = root
# Bind signal.
target_view.say.connect(say) # The former one is the signal of qml, and the latter one is from Python
# say() method.
sys.exit(app.exec())
main.qml:
import QtQuick 2.14
import QtQuick.Controls 2.14
import QtQuick.Window 2.12
Window {
visible: true
width: 600; height: 400
signal say(string s)
Button {
text: "hello"
onClicked: say(text)
}
}

Answer 1

Possible causes of the error are:
The path of the .qml is incorrect so the .qml is not loaded.
The loading of the .qml is not synchronous as you are assuming.
The .qml has some error (syntax for example)
Considering the above, the solution is:
import os
import sys
from pathlib import Path
from PySide6.QtCore import QCoreApplication, Qt, QUrl
from PySide6.QtQml import QQmlApplicationEngine
from PySide6.QtWidgets import QApplication
CURRENT_DIRECTORY = Path(__file__).resolve().parent
def say(s):
print(s)
if __name__ == "__main__":
app = QApplication()
engine = QQmlApplicationEngine()
filename = os.fspath(CURRENT_DIRECTORY / "main.qml")
url = QUrl.fromLocalFile(filename)
def handle_object_created(obj, obj_url):
if obj is None and url == obj_url:
QCoreApplication.exit(-1)
else:
root = engine.rootObjects()[0]
target_view = root
target_view.say.connect(say)
engine.objectCreated.connect(handle_object_created, Qt.QueuedConnection)
engine.load(url)
sys.exit(app.exec())
But anyway I don't prefer to use rootObjects but instead export a QObject:
import os
import sys
from pathlib import Path
from PySide6.QtCore import QCoreApplication, QObject, Qt, QUrl, Signal
from PySide6.QtQml import QQmlApplicationEngine
from PySide6.QtWidgets import QApplication
CURRENT_DIRECTORY = Path(__file__).resolve().parent
class Helper(QObject):
say = Signal(str)
def say(s):
print(s)
if __name__ == "__main__":
app = QApplication()
engine = QQmlApplicationEngine()
helper = Helper()
helper.say.connect(say)
engine.rootContext().setContextProperty("helper", helper)
filename = os.fspath(CURRENT_DIRECTORY / "main.qml")
url = QUrl.fromLocalFile(filename)
def handle_object_created(obj, obj_url):
if obj is None and url == obj_url:
QCoreApplication.exit(-1)
engine.objectCreated.connect(handle_object_created, Qt.QueuedConnection)
engine.load(url)
sys.exit(app.exec())
import QtQuick 2.14
import QtQuick.Controls 2.14
import QtQuick.Window 2.12
Window {
visible: true
width: 600
height: 400
Button {
text: "hello"
onClicked: helper.say(text)
}
}