Asynchronous call from QML to Python with callback

From QML, I'd like to:

1. Call a Python slot.
2. Pass along a callback.
3. Have that callback be run once the slot is complete.

I've tried this:

1. Register a context-property (Service)
2. Call Service.request("data", function (response) { console.log(response) }
3. In Python, the function is received as a QtQml.QJSValue
4. The function is called in a separate thread, after some expensive operation

However, the function only has an effect sometimes, and most of the time not at all or crashes the Python interpreter. If I remove the call to time.sleep(1), it is more likely to produce results.

Any ideas?

Here's a non-working implementation of the above

main.qml

import QtQuick 2.3

import "application.js" as App


Rectangle {
    id: appWindow
    width: 200
    height: 200
    Component.onCompleted: App.onLoad()
}

main.py

import sys
import time

import threading

from PyQt5 import QtCore, QtGui, QtQml, QtQuick


class Service(QtCore.QObject):
    def __init__(self, parent=None):
        super(Service, self).__init__(parent)

    @QtCore.pyqtSlot(str, str, QtCore.QVariant, QtQml.QJSValue)
    def request(self, verb, endpoint, data, cb):
        """Expensive call"""
        print verb, endpoint, data

        self.cb = cb

        def thread():
            time.sleep(1)
            event = QtCore.QEvent(1000)
            event.return_value = "expensive result"
            QtGui.QGuiApplication.postEvent(self, event)

        worker = threading.Thread(target=thread)
        worker.daemon = False
        worker.start()

        self.worker = worker

    def event(self, event):
        if event.type() == 1000:
            self.cb.call([event.return_value])

        return super(Service, self).event(event)


app = QtGui.QGuiApplication(sys.argv)
view = QtQuick.QQuickView()
context = view.rootContext()

service = Service()
context.setContextProperty("Service", service)

view.setSource(QtCore.QUrl("main.qml"))
view.show()
app.exec_()

application.js

"use strict";
/*global print, Service*/


function onLoad() {
    Service.request("POST", "/endpoint", {"data": "value"}, function (reply) {
        print(reply);
        print(reply);
        print(reply);
    });

    print("request() was made");
}

The implementation is adapted from here
https://github.com/ben-github/PyQt5-QML-CallbackFunction

Best,
Marcus