qt - How to connect Python and QML with PySide2?

Question

I want to write a simple desktop application on Ubuntu and I thought that an easy way was to use Qt with QML as GUI and Python as the language for the logic, since I am somewhat familiar with Python.

Now I am trying for hours to somehow connect the GUI and the logic, but it is not working. I managed the connection QML --> Python but not the other way around. I have Python classes which represent my data model and I added JSON encode and decode functions. So for now there is no SQL database involved. But maybe a direct connection between QML view and some database would make things easier?

So now some code.

QML --> Python

The QML file:

ApplicationWindow {

// main window
id: mainWindow
title: qsTr("Test")
width: 640
height: 480

signal tmsPrint(string text)

Page {
    id: mainView

    ColumnLayout {
        id: mainLayout

        Button {
            text: qsTr("Say Hello!")
            onClicked: tmsPrint("Hello!")
        }
    }
}    
}

Then I have my slots.py:

from PySide2.QtCore import Slot

def connect_slots(win):
    win.tmsPrint.connect(say_hello)

@Slot(str)
def say_hello(text):
    print(text)

And finally my main.py:

import sys
from controller.slots import connect_slots

from PySide2.QtWidgets import QApplication
from PySide2.QtQml import QQmlApplicationEngine 

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

    engine = QQmlApplicationEngine()
    engine.load('view/main.qml')

    win = engine.rootObjects()[0]
    connect_slots(win)

    # show the window
    win.show()
    sys.exit(app.exec_())

This works fine and I can print "Hello!". But is this the best way to do it or is it better to create a class with slots and use setContextProperty to be able to call them directly without adding additional signals?

Python --> QML

I cannot get this done. I tried different approaches, but none worked and I also don't know which one is the best to use. What I want to do is for example show a list of objects and offer means to manipulate data in the application etc.

1. include Javascript: I added an additional file application.js with a function just to print something, but it could probably be used to set the context of a text field etc. Then I tried to use QMetaObject and invokeMethod, but just got errors with wrong arguments etc.

Does this approach make any sense? Actually I don't know any javascript, so if it is not necessary, I would rather not use it.

2. ViewModel approach I created a file viewmodel.py

   from PySide2.QtCore import QStringListModel
   
   class ListModel(QStringListModel):
   
   def __init__(self):
        self.textlines = ['hi', 'ho']
        super().__init__()
   

And in the main.py I added:

model = ListModel()
engine.rootContext().setContextProperty('myModel', model)

and the ListView looks like this:

ListView {
            width: 180; height: 200

            model: myModel
            delegate: Text {
                text: model.textlines
            }
        }

I get an error "myModel is not defined", but I guess that it can't work anyway, since delegates only take one element and not a list. Is this approach a good one? and if yes, how do I make it work?

3. Is there a totally different approach to manipulate data in a QML view?

I appreciate your help! I know the Qt documentation but I am not happy with it. So maybe I am missing something. But PyQt seems to be way more popular than PySide2 (at least google searches seem to indicate that) and PySide references often use PySide1 or not the QML QtQuick way of doing things...

Answer 1

Your question has many aspects so I will try to be detailed in my answer and also this answer will be continuously updated because this type of questions are often asked but they are solutions for a specific case so I am going to take the liberty of giving it a general approach and be specific in the possible scenarios.

QML to Python:

Your method works because the type conversion in python is dynamic, in C++ it does not happen. It works for small tasks but it is not maintainable, the logic must be separated from the view so it should not be dependent. To be concrete, let's say that the printed text will be taken by the logic to perform some processing, then if you modify the name of the signal, or if the data does not depend on ApplicationWindow but on another element, etc. then you will have to change a lot connection code.

The recommended as you indicate is to create a class that is responsible for mapping the data you need your logic and embed it in QML, so if you change something in the view you just change the connection:

Example:

main.py

import sys

from PySide2.QtCore import QObject, Signal, Property, QUrl
from PySide2.QtGui import QGuiApplication
from PySide2.QtQml import QQmlApplicationEngine

class Backend(QObject):
    textChanged = Signal(str)

    def __init__(self, parent=None):
        QObject.__init__(self, parent)
        self.m_text = ""

    @Property(str, notify=textChanged)
    def text(self):
        return self.m_text

    @text.setter
    def setText(self, text):
        if self.m_text == text:
            return
        self.m_text = text
        self.textChanged.emit(self.m_text)   

if __name__ == '__main__':
    app = QGuiApplication(sys.argv)
    backend = Backend()

    backend.textChanged.connect(lambda text: print(text))
    engine = QQmlApplicationEngine()
    engine.rootContext().setContextProperty("backend", backend)
    engine.load(QUrl.fromLocalFile('main.qml'))
    if not engine.rootObjects():
        sys.exit(-1)
    sys.exit(app.exec_())

main.qml

import QtQuick 2.10
import QtQuick.Controls 2.1
import QtQuick.Window 2.2

ApplicationWindow {
    title: qsTr("Test")
    width: 640
    height: 480
    visible: true
    Column{
        TextField{
            id: tf
            text: "Hello"
        }
        Button {
            text: qsTr("Click Me")
            onClicked: backend.text = tf.text
        } 
    }
}

Now if you want the text to be provided by another element you just have to change the line: onClicked: backend.text = tf.text.

---

Python to QML:

1. I can not tell you what you did wrong with this method because you do not show any code, but I do indicate the disadvantages. The main disadvantage is that to use this method you must have access to the method and for that there are 2 possibilities, the first one is that it is a rootObjects as it is shown in your first example or searching through the objectName, but it happens that you initially look for the object, you get it and this is removed from QML, for example the Pages of a StackView are created and deleted every time you change pages so this method would not be correct.

2. The second method for me is the correct one but you have not used it correctly, unlike the QtWidgets that focus on the row and the column in QML the roles are used. First let's implement your code correctly.

First textlines is not accessible from QML since it is not a qproperty. As I said you must access through the roles, to see the roles of a model you can print the result of roleNames():

model = QStringListModel()
model.setStringList(["hi", "ho"])
print(model.roleNames())

output:

{
    0: PySide2.QtCore.QByteArray('display'),
    1: PySide2.QtCore.QByteArray('decoration'),
    2: PySide2.QtCore.QByteArray('edit'),
    3: PySide2.QtCore.QByteArray('toolTip'),
    4: PySide2.QtCore.QByteArray('statusTip'),
    5: PySide2.QtCore.QByteArray('whatsThis')
}

In the case that you want to obtain the text you must use the role Qt::DisplayRole, whose numerical value according to the docs is:

Qt::DisplayRole 0   The key data to be rendered in the form of text. (QString)

so in QML you should use model.display(or only display). so the correct code is as follows:

main.py

import sys

from PySide2.QtCore import QUrl, QStringListModel
from PySide2.QtGui import QGuiApplication
from PySide2.QtQml import QQmlApplicationEngine  

if __name__ == '__main__':
    app = QGuiApplication(sys.argv)
    model = QStringListModel()
    model.setStringList(["hi", "ho"])

    engine = QQmlApplicationEngine()
    engine.rootContext().setContextProperty("myModel", model)
    engine.load(QUrl.fromLocalFile('main.qml'))
    if not engine.rootObjects():
        sys.exit(-1)
    sys.exit(app.exec_())

main.qml

import QtQuick 2.10
import QtQuick.Controls 2.1
import QtQuick.Window 2.2

ApplicationWindow {
    title: qsTr("Test")
    width: 640
    height: 480
    visible: true
    ListView{
        model: myModel
        anchors.fill: parent
        delegate: Text { text: model.display }
    }
}

If you want it to be editable you must use the model.display = foo:

import QtQuick 2.10
import QtQuick.Controls 2.1
import QtQuick.Window 2.2

ApplicationWindow {
    title: qsTr("Test")
    width: 640
    height: 480
    visible: true
    ListView{
        model: myModel
        anchors.fill: parent
        delegate: 
        Column{
            Text{ 
                text: model.display 
            }
            TextField{
                onTextChanged: {
                    model.display = text
                }
            }
        }
    }
}

---

There are many other methods to interact with Python/C++ with QML but the best methods involve embedding the objects created in Python/C++ through setContextProperty.

As you indicate the docs of PySide2 is not much, it is being implemented and you can see it through the following link. What exists most are many examples of PyQt5 so I recommend you understand what are the equivalences between both and make a translation, this translation is not hard since they are minimal changes.