PyQt & QML garbage collection problem

pyqt_qml_gc_problem.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from __future__ import print_function
import os
import sys
import signal
from random import randint
from PyQt5.QtGui import QGuiApplication
from PyQt5.QtQuick import QQuickView
from PyQt5.QtCore import (Qt, QUrl, QObject, QVariant, QAbstractListModel,
                          pyqtSlot, pyqtProperty)


# This code can be run on both Python 2 and 3
#
# This example demonstrates the problem where DummyModel gets garbage
# collected by javascript. When the GUI button is pressed, you will see
# that DummyModel's __del__ and destroyed signal get triggered.
# After the model has been collected, listview will have missing items when
# scrolled. Garbage collection can also be triggered by rapidly scrolling the
# listview for about 10 seconds.
#
# By changing the two constants below you can try any combination of transfer
# methods and types of model items, however all 4 of them behave the same (I'm
# not considering the "nothing" transfer method) on the following setup:
#    Qt 5.3.1 64-bit
#    PyQt 5.3.2-snapshot-77bc10e785e2
#    sip 4.16.3-snapshot-53f490fe8f52
#    Ubuntu 14.04 64-bit
#    Python 3.4.0 64-bit


# change this to try out different ownership transfer methods
TRANSFER_METHOD = 2  # 0 - nothing, 1 - sip.transferto, 2 - setObjectOwnership

# change this to switch between model of integers and model of DummyObjects
MODEL_DATA_TYPE = 1  # 0 - integers, 1 - DummyObjects


if TRANSFER_METHOD == 0:
    print("\nWithout ownership transfer")

    def transfer_ownership(obj):
        pass

elif TRANSFER_METHOD == 1:
    print("\nOwnership transfer using sip.transferto")
    import sip

    def transfer_ownership(obj):
        sip.transferto(obj, obj)

elif TRANSFER_METHOD == 2:
    print("\nOwnership transfer using setObjectOwnership")
    from PyQt5.QtQml import QQmlEngine

    def transfer_ownership(obj):
        QQmlEngine.setObjectOwnership(obj, 1)

else:
    assert False, "wrong TRANSFER_METHOD value"




if MODEL_DATA_TYPE == 0:
    print("Model contains integers")

    def item_getter():
        return randint(1, 99)

elif MODEL_DATA_TYPE == 1:
    print("Model contains DummyObjects")

    class DummyObject(QObject):
        @pyqtProperty(str, constant=True)
        def something(self):
            self._something = "item #{}".format(randint(1, 99))
            return self._something

    def item_getter():
        d = DummyObject()
        transfer_ownership(d)
        return d

else:
    assert False, "wrong MODEL_DATA_TYPE value"



class DummyModel(QAbstractListModel):
    ROLES = ("hello",)
    _ROLE_MAP = {}

    def _setup_roles(self):
        keys = range(Qt.UserRole + 1, Qt.UserRole + 1 + len(self.ROLES))
        self._ROLE_MAP = dict(zip(keys, self.ROLES))

    def roleNames(self):
        return self._ROLE_MAP

    def __init__(self, parent=None):
        super(DummyModel, self).__init__(parent)
        self._setup_roles()

        self._size = 20

        def bye():
            print("\n  DummyModel DESTROYED")

        self.destroyed.connect(bye)

    def data(self, qindex, role):
        i = qindex.row()
        # these three cases never happen, left them in just in case
        if not qindex.isValid():
            print("Invalid index, returning empty QVariant")
            return QVariant()
        if role not in self._ROLE_MAP:
            print("Invalid role, returning empty QVariant")
            return QVariant()
        if i < 0 or i > self._size:
            print("index {} out of bounds, returning empty QVariant".format(i))
            return QVariant()

        return item_getter()

    @pyqtSlot(result=int)
    def rowCount(self, parent):
        if parent.isValid():
            # this never happens
            raise RuntimeError("Model is a flat list, it can't have a parent")
        return self._size

    def __del__(self):
        print("\n  DummyModel DELETED")



class GUIEntryPoint(QObject):

    @pyqtSlot(result=QVariant)
    def get_model(self):
        d = DummyModel()
        transfer_ownership(d)
        return d

    @pyqtProperty(int, constant=True)
    def model_data_type(self):
        return MODEL_DATA_TYPE



app = None


def run():
    global app
    signal.signal(signal.SIGINT, signal.SIG_DFL)

    app = QGuiApplication(sys.argv)

    qml_url = QUrl.fromLocalFile(os.path.join('ui.qml'))
    view = QQuickView()
    gep = GUIEntryPoint()
    view.rootContext().setContextProperty('BE', gep)
    view.setSource(qml_url)

    view.show()
    sys.exit(app.exec_())


if __name__ == '__main__':
    run()

ui.qml

import QtQuick 2.2
import QtQuick.Controls 1.2


Item {
    width: 300
    height: 200

    ListView {
        width: 50
        height: 100
        cacheBuffer: 0
        spacing: 1
        model: BE.get_model()
        delegate: Rectangle {
            width: 60
            height: 15
            color: "gray"
            Text { text: BE.model_data_type == 0 ? hello : hello.something }
        }
    }


    Button {
        x: 80
        y: 30
        text: "Call javascript gc()"

        onClicked: gc()
    }
}

Check out revision #1 for a bit simpler code