Bug report: latest pyqt5 from pypi breaks pyface and traitsui

Environment

OS: Windows/Linux Python version: 3.7/3.8/3.9/3.10 Toolkit: Qt Qt API: PyQt5

Description

The latest pyqt5 release on pypi (5.15.8) seems to have changed its API and as a result breaks pyface and as a consequence also traitsui because it relies on pyqt imports made by pyface. I don't see any mention of this in the release notes: https://www.riverbankcomputing.com/news/PyQt_v5.15.8_Released

The last known working release of pyqt5 is 5.15.7 .

Steps to Reproduce

create fresh venv
pip install traitsui pyface[pyqt5]

Taking an example from the traitsui docs and enforcing the toolkit produces a traceback pointing at the root cause:

if __name__ == '__main__':
from traits.etsconfig.api import ETSConfig
ETSConfig.toolkit = 'qt'

from traits.api import HasTraits, Int, Str
from traitsui.api import Item, View

class SimpleEmployee(HasTraits):
    first_name = Str()
    last_name = Str()
    department = Str()
    employee_number = Str()
    salary = Int()

view1 = View(
    Item(name='first_name'),
    Item(name='last_name'),
    Item(name='department'),
)

sam = SimpleEmployee()
sam.configure_traits(view=view1)

Results in this traceback


C:\dev\.venvpyqt5\Scripts\python.exe C:\dev\pyface_issue\traitsui-issue.py 
Traceback (most recent call last):
File "C:\dev\.venvpyqt5\lib\site-packages\pyface\base_toolkit.py", line 219, in import_toolkit
toolkit_object = plugin.load()
File "C:\Python39\lib\importlib\metadata.py", line 86, in load
module = import_module(match.group('module'))
File "C:\Python39\lib\importlib\__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 850, in exec_module
File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
File "C:\dev\.venvpyqt5\lib\site-packages\pyface\ui\qt4\init.py", line 19, in <module>
from .gui import GUI
File "C:\dev\.venvpyqt5\lib\site-packages\pyface\ui\qt4\gui.py", line 18, in <module>
from pyface.qt import QtCore, QtGui
File "C:\dev\.venvpyqt5\lib\site-packages\pyface\qt\QtCore.py", line 30, in <module>
from PyQt5.Qt import QCoreApplication
ImportError: cannot import name 'QCoreApplication' from 'PyQt5.Qt' (C:\dev\.venvpyqt5\lib\site-packages\PyQt5\Qt.pyd)

The above exception was the direct cause of the following exception:

Traceback (most recent call last): File "C:\dev\pyface_issue\traitsui-issue.py", line 9, in from traitsui.api import Item, View File "C:\dev.venvpyqt5\lib\site-packages\traitsui\api.py", line 257, in from .editors.api import ArrayEditor File "C:\dev.venvpyqt5\lib\site-packages\traitsui\editors__init.py", line 16, in from .api import ArrayEditor File "C:\dev.venvpyqt5\lib\site-packages\traitsui\editors\api.py", line 96, in from .code_editor import CodeEditor File "C:\dev.venvpyqt5\lib\site-packages\traitsui\editors\code_editor.py", line 21, in class CodeEditor(EditorFactory): File "C:\dev.venvpyqt5\lib\site-packages\traitsui\editors\code_editor.py", line 32, in CodeEditor mark_color = Color(0xECE9D8) File "C:\dev.venvpyqt5\lib\site-packages\traits\trait_factory.py", line 40, in call return self.maker_function(*args, *metadata) File "C:\dev.venvpyqt5\lib\site-packages\traitsui\toolkit_traits.py", line 43, in ColorTrait return toolkit().color_trait(args, **traits) File "C:\dev.venvpyqt5\lib\site-packages\traitsui\toolkit.py", line 110, in toolkit _toolkit = find_toolkit("traitsui.toolkits") File "C:\dev.venvpyqt5\lib\site-packages\pyface\base_toolkit.py", line 265, in find_toolkit return import_toolkit(ETSConfig.toolkit, entry_point) File "C:\dev.venvpyqt5\lib\site-packages\pyface\base_toolkit.py", line 219, in import_toolkit toolkit_object = plugin.load() File "C:\Python39\lib\importlib\metadata.py", line 86, in load module = import_module(match.group('module')) File "C:\Python39\lib\importlib\init.py", line 127, in import_module return _bootstrap._gcd_import(name[level:], package, level) File "C:\dev.venvpyqt5\lib\site-packages\traitsui\qt4\init__.py", line 35, in from . import toolkit File "C:\dev.venvpyqt5\lib\site-packages\traitsui\qt4\toolkit.py", line 33, in from pyface.toolkit import toolkit_object as pyface_toolkit File "C:\dev.venvpyqt5\lib\site-packages\pyface\toolkit.py", line 23, in toolkit = toolkit_object = find_toolkit("pyface.toolkits") File "C:\dev.venvpyqt5\lib\site-packages\pyface\base_toolkit.py", line 265, in find_toolkit return import_toolkit(ETSConfig.toolkit, entry_point) File "C:\dev.venvpyqt5\lib\site-packages\pyface\base_toolkit.py", line 231, in import_toolkit raise RuntimeError(msg) from toolkit_exception RuntimeError: No pyface.toolkits plugin could be loaded for qt

5. The root cause seems to be that
```python
from PyQt5.Qt import QCoreApplication

is no longer possible. This seems to work instead:

from PyQt5.QtCore import QCoreApplication

If you don't force the toolkit and instead rely on autodiscovery you get this rather unspecific traceback:

Traceback (most recent call last):
File "C:\dev\pyface_issue\traitsui-issue.py", line 24, in <module>
sam.configure_traits(view=view1)
File "C:\dev\.venvpyqt5\lib\site-packages\traits\has_traits.py", line 2164, in configure_traits
rc = toolkit().view_application(
File "C:\dev\.venvpyqt5\lib\site-packages\traitsui\toolkit.py", line 238, in view_application
raise NotImplementedError(
NotImplementedError: the 'null' toolkit does not implement this method

enthought / pyface