How to run PyQt5 GUIs in non-blocking threads?

Question

I have a PyQt5 GUI class that I want to be able to create multiple instances of either from an interactive console or normal run. I need these GUIs to be non-blocking so that they can be used while subsequent code runs.

I've tried calling app.exec__() in separate threads for each GUI like this answer, but the program sometimes crashes as the comment on the answer warned it would:
Run pyQT GUI main app in seperate Thread

And now I'm trying to get the code below working which I made based on this answer:
Run Pyqt GUI main app as a separate, non-blocking process

But when I run it the windows pop and and immediately disappear

import sys
from PyQt5 import QtWidgets, QtGui, QtCore
import time

class MainWindow(QtWidgets.QWidget):
    def __init__(self):
        # call super class constructor
        super(MainWindow, self).__init__()
        # build the objects one by one
        layout = QtWidgets.QVBoxLayout(self)
        self.pb_load = QtWidgets.QPushButton('Load')
        self.pb_clear= QtWidgets.QPushButton('Clear')
        self.edit = QtWidgets.QTextEdit()
        layout.addWidget(self.edit)
        layout.addWidget(self.pb_load)
        layout.addWidget(self.pb_clear)
        # connect the callbacks to the push-buttons
        self.pb_load.clicked.connect(self.callback_pb_load)
        self.pb_clear.clicked.connect(self.callback_pb_clear)

    def callback_pb_load(self):
        self.edit.append('hello world')
    def callback_pb_clear(self):
        self.edit.clear()

def show():
    app = QtWidgets.QApplication.instance()
    if not app:
        app = QtWidgets.QApplication(sys.argv)
    win = MainWindow()
    win.show()

if __name__ == '__main__':
    show()
    show()

EDIT - I don't see how this question is a duplicate. The 'duplicate' questions are only slightly related and don't provide solutions to my problem at all.

I want to be able to create multiple instances of a GUI (MainWindow in my example) by calling the show() function from either an interactive session or script, and I want those windows to stay on my screen while subsequent code is running.

EDIT2 - When I run the code as a script I can do what I want by using multiprocessing, see this demo:
https://www.screencast.com/t/5WvJNVSLm9OR

However I still need help because I want it to also work in interactive Python console sessions, and multiprocessing does not work in that case.

Answer 1

It isn't necessary to use separate threads or processes for this. You just need a way to maintain a reference to each new window when importing the script in a python interactive session. A simple list can be used for this. It is only necessary to explictly start an event-loop when running the script from the command-line; in an interactive session, it will be handled automatically by PyQt.

Here is an implementation of this approach:

...
_cache = []

def show(title=''):
    if QtWidgets.QApplication.instance() is None:
        _cache.append(QtWidgets.QApplication(sys.argv))
    win = MainWindow()
    win.setWindowTitle(title)
    win.setAttribute(QtCore.Qt.WA_DeleteOnClose)
    win.destroyed.connect(lambda: _cache.remove(win))
    _cache.append(win)
    win.show()

if __name__ == '__main__':

    show('Foo')
    show('Bar')

    sys.exit(QtWidgets.QApplication.instance().exec_())

Answer 2

This is a minor addendum to @ekhumoro's answer. I don't have enough reputation to only add a comment so I had to write this as an answer.

@ekhumoro's answer almost fully answers @Esostack's question, but doesn't work in the Ipython console. After many hours of searching for the answer to this question myself, I came across a comment from @titusjan in a three year old thread (here) also responding to a good answer from @ekhumoro. The missing part to @ekhumoro's answer which results in the gui windows freezing for Ipython specifically is that Ipython should be set to use the qt gui at launch or once running.

---

To make this work with Ipython:

Launch Ipython with ipython --gui=qt5

In a running Ipython console run the magic command %gui qt5

To fix it from a Python script you can run this function

def fix_ipython():
    from IPython import get_ipython

    ipython = get_ipython()
    if ipython is not None:
        ipython.magic("gui qt5")