QML not taking ownership of object received from PyQt slot

Question

What is the correct way to call a Python function from QML that will instantiate and return an object to QML, and have QML side be responsible for the lifetime of the object? The problem I'm facing is that object gets garbage collected before it even reaches QML.

Note that I don't want to keep explicit references to the object on the Python side, and according to the following quote from the documentation what I'm looking for should even be the default behavior:

When data is transferred from C++ to QML, the ownership of the data always remains with C++. The exception to this rule is when a QObject is returned from an explicit C++ method call: in this case, the QML engine assumes ownership of the object...

I have created a minimal runnable example that demonstrates the problem (gist link):

main.py:

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

import os
import sys
import signal
from PyQt5.QtGui import QGuiApplication
from PyQt5.QtQuick import QQuickView
from PyQt5.QtCore import QUrl, QObject, QVariant, pyqtSlot
from PyQt5.QtQml import QQmlEngine, qmlRegisterType


class Dummy(QObject):

    def __del__(self):
        print("Deleted")


class GUIEntryPoint(QObject):

    @pyqtSlot(result=QVariant)
    def get_foo(self):
        foo = Dummy()
        print("Created {}".format(foo))
        print("Ownership after instantiation: {}".format(QQmlEngine.objectOwnership(foo)))
        #QQmlEngine.setObjectOwnership(foo, 1)  # has no effect
        return foo

    @pyqtSlot(QVariant)
    def print_foo(self, foo):
        print("{}, ownership: {}".format(foo, QQmlEngine.objectOwnership(foo)))


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

    app = QGuiApplication(sys.argv)

    # these don't seem to make a difference
    qmlRegisterType(GUIEntryPoint, 'GUIEntryPoint', 1, 0, 'GUIEntryPoint')
    qmlRegisterType(Dummy, 'Dummy', 1, 0, 'Dummy')

    qml_url = QUrl.fromLocalFile(os.path.join('zzz.qml'))
    view = QQuickView()

    gep = GUIEntryPoint()
    view.rootContext().setContextProperty('BE', gep)
    view.setSource(qml_url)

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


if __name__ == '__main__':
    run()

zzz.qml:

import QtQuick 2.2


Rectangle {
    id: rootItem
    width: 640
    height: 400

    property var pleaseKeepMe: BE.get_foo()

    Component.onCompleted: {
        console.log("Completed");

        BE.print_foo(pleaseKeepMe);  // prints None, it has been deleted

        console.log("creating another");
        var x = BE.get_foo();
        // at this point x has already been deleted on the Python side
        console.log("created another");
        BE.print_foo(x);  // prints None

        console.log("\n\nPress CTRL-C to exit");
    }
}

score 7 · Answer 1 · edited May 23 '17 at 10:24

In short: using SIP Python API to transfer ownership to C++ while keeping the Python "part" of a QObject alive seemingly solves the issue, which is indeed an ownership one, as covered at (also see the comments in sources below):

On SIP API, see:

Also, some nice general advice is at:

https://stackoverflow.com/a/11945184/110118

A slightly modified example from the question, Python source:

from PyQt5.QtGui import QGuiApplication
from PyQt5.QtQuick import QQuickView
from PyQt5.QtCore import QUrl, QObject, QVariant, pyqtSlot
from PyQt5.QtQml import QQmlEngine, qmlRegisterType
import sip

def ownership(obj):
    jsOwned = QQmlEngine.objectOwnership(obj) == QQmlEngine.JavaScriptOwnership
    return "Javascript" if jsOwned else "Cpp"  

def onDummytDestroyed():
    print "Dummy destroyed"

class Dummy(QObject):

    i = 0

    def __init__(self):
        #let's not forget to properly initialize the base part
        super(Dummy, self).__init__()
        self.i = Dummy.i
        Dummy.i = Dummy.i + 1

    def __del__(self):
        #let's track when the Python part gets destroyed 
        #  (or, rather, "finalized", or whatever the correct term is :) )
        print "Dummy.__del__(), self.i:", self.i

    @pyqtSlot(result="QString")
    def get_str(self):
        #let's have a Python (not C++) implemented method in the class
        return "Dummy string #" + str(self.i)

class GUIEntryPoint(QObject):

    def __init__(self):
        #same as with Dummy.__init__
        super(GUIEntryPoint, self).__init__()

    @pyqtSlot(result=QVariant) #would also work
    #@pyqtSlot(result=QObject)
    def get_foo(self):
        print "GUIEntryPoint.get_foo():"
        foo = Dummy()
        foo.setObjectName("Foo")

        #let's track when the C++ part gets destroyed
        foo.destroyed.connect(onDummytDestroyed)

        #this would destroy the Python part of foo as soon as the reference
        #goes out of scope (the C++ part stay alive and will be properly accessible from QML,
        #but attempts to access Python-implemented methods like Dummy.get_str()
        #would cause a TypeError hinting that there is no such method) 
        #sip.transferto(foo, None)

        #this works as expected, with both Python (and then C++) parts of foo
        #being destroyed whenever the QML GC decides to get rid of the object
        #(see program output, notice how GC is silent for a while)  
        sip.transferto(foo, foo)

        #this is indeed not needed, as proper ownership will be set
        #when the object passes to the QML land through the slot invocation
        #QQmlEngine.setObjectOwnership(foo, QQmlEngine.JavaScriptOwnership)
        print "    initial ownership: ", ownership(foo)

        return foo

    @pyqtSlot(QObject) #would also work
    #@pyqtSlot(QVariant) 
    def print_foo(self, foo):
        print "GUIEntryPoint.print_foo():"
        print "    objectName: ", foo.objectName()
        print "    ownership when on QML side: ", ownership(foo)

app = QGuiApplication([])
view = QQuickView()

#not needed, indeed, as both context objects (and properties),
#and QObject descendants passed to QML side are perfectly accessible
#(that is, their properties and invokables / slots are) 
#without any additional steps
#(instantiating types declaratively would require registering them first, though,
#and having properties of custom types requires metatypes to be declared)
#qmlRegisterType(GUIEntryPoint, 'GUIEntryPoint', 1, 0, 'GUIEntryPoint')
#qmlRegisterType(Dummy, 'Dummy', 1, 0, 'Dummy')

gep = GUIEntryPoint()
view.rootContext().setContextProperty('BE', gep)

view.setSource(QUrl.fromLocalFile("pyqt-ownership-test.qml"))
view.show()

app.exec_()

QML source (save as pyqt-ownership-test.py):

import QtQuick 2.1

Rectangle {
    width: 300
    height: 200
    color: "red"
    border.width: 10

    Component.onCompleted: {
        console.log("Component.onCompleted():")

        //increase the number to 1000 to see GC stepping in
        //while the loop is still running
        //(notice how destruction is not being reported for some objects,
        //for example, #249 (the one created last),
        //but this looks like a Qt/QML logging issue)
        for (var i = 0; i < 250; i++) {
            console.log("#" + i)
            var foo = BE.get_foo()

            console.log("    foo: " + foo)
            console.log("    foo.objectName: " + foo.objectName)
            console.log("    foo.get_str(): " + foo.get_str())

            console.log("    print_foo():")
            BE.print_foo(foo)
        }
    }
}

Program output (stripped in the middle to accomodate SO answer size limit):

Component.onCompleted():
#0
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x22903d0, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #0
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#1
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x2290d20, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #1
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#2
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x2291080, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #2
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#3
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x22911e0, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #3
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#4
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x2291e90, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #4
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#5
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x2292b70, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #5
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#6
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x2293420, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #6
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#7
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x2296e30, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #7
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#8
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x22983b0, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #8
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#9
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x2299150, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #9
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#10
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x2299e60, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #10
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#11
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x229ab70, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #11
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript

<...some lines skipped here...>

#245
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x23007b0, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #245
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#246
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x22fe4d0, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #246
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#247
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x22fc5c0, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #247
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#248
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x22fa730, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #248
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
#249
GUIEntryPoint.get_foo():
    initial ownership:  Cpp
    foo: Dummy(0x22f57b0, "Foo")
    foo.objectName: Foo
    foo.get_str(): Dummy string #249
    print_foo():
GUIEntryPoint.print_foo():
    objectName:  Foo
    ownership when on QML side:  Javascript
Dummy.__del__(), self.i: 214
Dummy destroyed
Dummy.__del__(), self.i: 213
Dummy destroyed
Dummy.__del__(), self.i: 212
Dummy destroyed
Dummy.__del__(), self.i: 211
Dummy destroyed
Dummy.__del__(), self.i: 210
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Dummy.__del__(), self.i: 11
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Dummy.__del__(), self.i: 9
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Dummy.__del__(), self.i: 8
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Dummy.__del__(), self.i: 6
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Dummy.__del__(), self.i: 5
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Dummy.__del__(), self.i: 4
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Dummy.__del__(), self.i: 3
Dummy destroyed
Dummy.__del__(), self.i: 2
Dummy destroyed
Dummy.__del__(), self.i: 1
Dummy destroyed
Dummy.__del__(), self.i: 0
Dummy destroyed

Thanks for the detailed answer. I don't get to see the destruction printouts on application close, is that something to be concerned about? I would expect a cleanup to happen when the application is quitting. Also, I occasionally get a "Segmentation fault" on exit, on Python 3.4 it happens in about 1 in 20 runs, on Python 2.7 it's more frequent, maybe 1 in 5. Is that also something to be concerned about? — bgr, Jul 14 '14 at 11:14
The latter issue, I believe, is sadly usual with PyQt apps (even simple ones exit with a segfault sometimes -- like a trivial QWidget shown with a QApplication starting afterwards), this may be tied to the order of object destruction at exit (see https://www.google.ru/search?q=pyqt+segfault+at+exit&client=ubuntu&channel=cs&oq=pyqt+segfault+at+exit&aqs=chrome.0.69i57j69i62l2.7298j0&sourceid=chrome&ie=UTF-8), and actually something PyQt docs happen to talk about (http://pyqt.sourceforge.net/Docs/PyQt5/pyqt4_differences.html#object-destruction-on-exit), claiming it to be solved though. — mlvljr, Jul 14 '14 at 13:03
Speaking of __del__() and onDestroyed() being logged to console, this should happen as soon as QML GC finds enough spare time to do that (after onCompleted() exits in QML, i.e., and not at exit, at least in my experiments). Would be interesting to see the logs you get (note, also, I was using Python 2, not 3). — mlvljr, Jul 14 '14 at 13:07

QML not taking ownership of object received from PyQt slot

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