Warn when the Qt application quits 'early'

[altendky]

Follows from #262.

[codecov]

Codecov Report

Merging #267 (c1a3a97) into main (2cc53c0) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #267   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           35        35           
  Lines         2023      2036   +13     
  Branches       138       139    +1     
=========================================
+ Hits          2023      2036   +13     

Impacted Files	Coverage Δ
qtrio/__init__.py	100.00% <ø> (ø)
qtrio/_core.py	100.00% <100.00%> (ø)
qtrio/_exceptions.py	100.00% <100.00%> (ø)
qtrio/_tests/test_core.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2cc53c0...c1a3a97. Read the comment docs.

[altendky]

@vxgmichel, would you like to review or test this before I merge? Does it fulfill your interest in a notification?

[vxgmichel]

Looks good to me 👍

I ran a quick test with the problem I originally faced with #262. I was surprised to not see the warning appear before I realized two Qt applications were created:

• one in qtrio.run
• and another one in the main async function

Weirdly enough, this caused the aboutToQuit signal of the first application to not be emitted. Once I made sure the app was created before running qtrio.run, the PR worked as expected.

[altendky]

That's... super weird. The Qt application is a singleton. You shouldn't be able to have two.

>>> from PySide2 import QtCore
>>> a1 = QtCore.QCoreApplication([])
>>> a2 = QtCore.QCoreApplication([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: Please destroy the QCoreApplication singleton before creating a new QCoreApplication instance.

But with PyQt5 it does let you do that... uh...

>>> from PyQt5 import QtCore
>>> a1 = QtCore.QCoreApplication([])
>>> a2 = QtCore.QCoreApplication([])
>>> a1
<PyQt5.QtCore.QCoreApplication object at 0x7f3c88b0e550>
>>> a2
<PyQt5.QtCore.QCoreApplication object at 0x7f3c8584d4c0>

And PyQt6 (6.1.1) still does.

>>> from PyQt6 import QtCore
>>> a1 = QtCore.QCoreApplication([])
>>> a2 = QtCore.QCoreApplication([])
>>> a1
<PyQt6.QtCore.QCoreApplication object at 0x7f73ce8b0550>
>>> a2
<PyQt6.QtCore.QCoreApplication object at 0x7f73ce7e80d0>

[altendky]

https://doc.qt.io/qt-6/qcoreapplication.html

For non-GUI application that uses Qt, there should be exactly one QCoreApplication object.

So it apparently isn't a singleton at the C++ level, you are just supposed to not create multiple. Someone did some experimentation for us as to what the implications of having multiple applications are. I recreated the signal separation issue.

from PyQt6 import QtCore

a1 = QtCore.QCoreApplication([])
a2 = QtCore.QCoreApplication([])

def f1():
    print("f1")

def f2():
    print("f2")

a1.applicationNameChanged.connect(f1)
a2.applicationNameChanged.connect(f2)

print("---")
a1.applicationNameChanged.emit()
print("---")
a2.applicationNameChanged.emit()
print("---")

---
f1
---
f2
---

Anyways, I think I'll leave dealing with multiple application instances as a separate activity, if it gets handled at all. The shutdown aspects are a thing I think are appropriate for QTrio to make efforts on since QTrio is necessarily tied up in the lifetime of the application. Creating multiple applications is just a thing you shouldn't do regardless.