Сигнал с именованным параметром

Вы не можете воспроизвести это так, если вы хотите реализовать проект, вы можете заставить слот возвращать значение:

main.py

from PySide2 import QtCore, QtGui, QtQml


class Calculator(QtCore.QObject):
    # Slot for summing two numbers
    @QtCore.Slot(int, int, result=int)
    def sum(self, arg1, arg2):
        return arg1 + arg2

    # Slot for subtraction of two numbers
    @QtCore.Slot(int, int, result=int)
    def sub(self, arg1, arg2):
        return arg1 - arg2


if __name__ == "__main__":
    import os
    import sys

    # Create an instance of the application
    app = QtGui.QGuiApplication(sys.argv)
    # Create QML engine
    engine = QtQml.QQmlApplicationEngine()
    # Create a calculator object
    calculator = Calculator()
    # And register it in the context of QML
    engine.rootContext().setContextProperty("calculator", calculator)
    # Load the qml file into the engine
    file = os.path.join(os.path.dirname(os.path.realpath(__file__)), "main.qml")
    engine.load(file)

    engine.quit.connect(app.quit)
    sys.exit(app.exec_())

main.qml

import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Layouts 1.2
 
ApplicationWindow {
    visible: true
    width: 640
    height: 240
    title: qsTr("PyQt5 love QML")
    color: "whitesmoke"
 
    GridLayout {
        anchors.top: parent.top
        anchors.left: parent.left
        anchors.right: parent.right
        anchors.margins: 9
 
        columns: 4
        rows: 4
        rowSpacing: 10
        columnSpacing: 10
 
        Text {
            text: qsTr("First number")
        }
 
        // Input field of the first number
        TextField {
            id: firstNumber
        }
 
        Text {
            text: qsTr("Second number")
        }
 
        // Input field of the second number
        TextField {
            id: secondNumber
        }
 
        Button {
            height: 40
            Layout.fillWidth: true
            text: qsTr("Sum numbers")
 
            Layout.columnSpan: 2
 
            onClicked: {
                // Invoke the calculator slot to sum the numbers
                sumResult.text = calculator.sum(firstNumber.text, secondNumber.text)
            }
        }
 
        Text {
            text: qsTr("Result")
        }
 
        // Here we see the result of sum
        Text {
            id: sumResult
        }
 
        Button {
            height: 40
            Layout.fillWidth: true
            text: qsTr("Subtraction numbers")
 
            Layout.columnSpan: 2
 
            onClicked: {
                // Invoke the calculator slot to subtract the numbers
                subResult.text = calculator.sub(firstNumber.text, secondNumber.text)
            }
        }
 
        Text {
            text: qsTr("Result")
        }
 
        // Here we see the result of subtraction
        Text {
            id: subResult
        }
    }
}

Примечание. Для PyQt5 измените Slot на pyqtSlot.