@pyqtslot return value is undefined

Question

I have a QML application that uses settings. So I created a python slot that reads the settings.toml, and returns the value. I've setup the context properties correctly and I can call other functions without a return value from QML without a problem.

...
class Settings(QObject)
    @pyqtSlot(str, str)
    def getSettings(self, category, key):
        try:
            with open("settings.toml", "r") as settings:
                toml_object = toml.load(settings)
            return str(toml_object[category][key])
        except FileNotFoundError:
            self.settingsFileNotFound.emit()
        except toml.TomlDecodeError:
            self.settingsError.emit()
        except BaseException:
            self.fatalError.emit()
...

The settings.toml looks like this.

...
[last-used-font-settings]
font = "Arial"
... 

When printing the function from python, it works as expected.

...
print(Settings.getSettings("last-used-font-settings", "font")) # Returns Arial
...

But when logging from qml:

...   
Component.onCompleted: console.log(Settings.getSettings("last-used-font-settings", "font"))
/* Returns qml: undefined */
...

How to solve this? I'm not very good at creating @pyqtProperty, but if I manage to create one, definitely I do not want to create a property for every value!

And signals wont work for this purpose you see. :(

Answer 1

If you want to return a value from python then you must use "result" in pyqtSlot:

import os

from PyQt5.QtCore import pyqtProperty, pyqtSlot, QObject, QUrl
from PyQt5.QtGui import QGuiApplication
from PyQt5.QtQml import QQmlApplicationEngine

import toml

CURRENT_DIR = os.path.dirname(os.path.realpath(__file__))


class Settings(QObject):
    def __init__(self, parent=None):
        super().__init__(parent)
        self._error_string = ""

    @pyqtProperty(str, constant=True)
    def errorString(self):
        return self._error_string

    @pyqtSlot(result=bool)
    def hasError(self):
        return bool(self.errorString)

    @pyqtSlot(str, str, result=str)
    def getSettings(self, category, key):
        self._error_string = ""
        error_string = ""
        value = ""
        try:
            with open(os.path.join(CURRENT_DIR, "settings.toml"), "r") as settings:
                toml_object = toml.load(settings)
                value = str(toml_object[category][key])
        except FileNotFoundError:
            error_string = "FileNotFoundError"
        except toml.TomlDecodeError:
            error_string = "TomlDecodeError"
        except Exception as e:
            error_string = str(e)
        self._error_string = error_string
        return value


if __name__ == "__main__":
    import sys

    app = QGuiApplication(sys.argv)
    engine = QQmlApplicationEngine()
    settings = Settings()
    engine.rootContext().setContextProperty("Settings", settings)

    filename = os.path.join(CURRENT_DIR, "main.qml")
    engine.load(QUrl.fromLocalFile(filename))

    if not engine.rootObjects():
        sys.exit(-1)
    sys.exit(app.exec())

import QtQuick 2.12
import QtQuick.Controls 2.12

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    Component.onCompleted: {
        var value = Settings.getSettings("last-used-font-settings", "font")
        if(Settings.hasError()){
            console.error(Settings.errorString)
        }
        else{
            console.log(value)
        }
    }
}