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Type: Bug
Resolution: Fixed
Priority: Not Evaluated
Fix Version/s: 6.5.2, 6.6.0
Affects Version/s: 6.5.0
Component/s: PySide
Labels:
None
Environment:
Python 3.8 and Python 3.10, example above tested on Windows 10, however our CI shows the same problem in MacOS and Ubuntu.

Platform/s:

Linux/X11, macOS, Windows
Commits:
baedbe835 (dev), 28ddc0e50 (6.5)

Description

Hi,

Chained exceptions raised inside virtual methods seem to have their {}{}clause__ attribute replaced by a str instance.

Consider this example:

import sys

from PySide6.QtCore import QObject, QEvent
from PySide6.QtWidgets import QApplication


class Receiver(QObject):

    def event(self, ev):
        try:
            try:
                raise RuntimeError('original error')
            except RuntimeError:
                raise ValueError('mistakes were made')
        except:
            print(repr(sys.exc_info()[1].__context__))
            raise


app = QApplication([])

v = Receiver()
app.sendEvent(v, QEvent(QEvent.Type.User))
app.sendEvent(v, QEvent(QEvent.Type.User))
app.processEvents()

This gives this output:

λ python tests\foo.py
RuntimeError('original error')
TypeError: print_exception(): Exception expected for value, str found

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "e:\projects\pytest-qt\tests\foo.py", line 23, in <module>
    app.sendEvent(v, QEvent(QEvent.Type.User))
  File "e:\projects\pytest-qt\tests\foo.py", line 14, in event
    raise ValueError('mistakes were made')
ValueError: mistakes were made

So the first print {{RuntimeError('original error') tells us that the _context is correctly set when the exception occurs (as it should), but some time later the __context_ attribute is changed to `"Delayed ValueError exception:"` (`a str`), which breaks traceback formatting functions.}}

As far as I can tell this behavior is independent of the Python version used, in this case I tested with Python 3.8 and Python 3.10.

The same example using `PyQt6` gives the expected results (we just need to replace `PySide6` by `PyQt6` in the previous example):

λ python tests\foo.py
RuntimeError('original error')
Traceback (most recent call last):
File "e:\projects\pytest-qt\tests\foo.py", line 12, in event
raise RuntimeError('original error')
RuntimeError: original error

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "e:\projects\pytest-qt\tests\foo.py", line 14, in event
raise ValueError('mistakes were made')
ValueError: mistakes were made

So I think the problem is in `PySide6` and how it is handling chained exceptions that originated from a C++ virtual method call.

------

This was detected in pytest-dev/pytest-qt#488.

Here's a link to our CI: https://github.com/pytest-dev/pytest-qt/actions/runs/4928752628/jobs/8807510393