Widgets are not updated - is this a bug?

[Phil Thompson]

On 15/09/2020 04:01, Jeremy Katz wrote:
> On 24/Jun/20 23:46, Christian ARNAUD wrote:
>> Hi,
>> 
>>  
>> 
>> I ran in a strange behavior where widgets in the view aren’t properly
>> refreshed on change.
>> 
>> Here is the code, developed to show the problem:
>> 
> [ code omitted ]  
>> 
>> If the pushbutton is clicked with the mouse or with the space key 
>> (after
>> getting the focus with Tab), the text in the lineedit is not 
>> refreshed.
>> If a repaint is forced, it is.
>> 
>> If the pushbutton is clicked with the enter key (after getting the 
>> focus
>> with Tab), the text in the lineedit is correctly refreshed.
>> 
>>  
>> 
>> Any idea?
>> 
>>  
>> 
>> My env:
>> 
>> OSX Catalina 10.15.5
>> Python 3.6
>> PyQt: 15.0
>> 
> 
> I'm having similar issues with macOS 10.15.6, Python 3.6.3, Qt 5.15.0,
> and PyQt 5.15.0. This doesn't appear to be an issue on Windows 10 or
> Linux/X11, and doesn't occur on the same version of macOS with a nearly
> identical C++ program.
> 
> The QTextEdit is correctly painted as soon as focus is transferred to
> another window via mouse click, command-tab, etc. Scheduling the
> operation that eventually leads to a repaint for the next iteration of
> the event loop also works.
> 
> import sys
> import PyQt5.QtWidgets as W
> import PyQt5.QtCore as C
> 
> app = W.QApplication(sys.argv)
> w = W.QWidget()
> layout = W.QVBoxLayout()
> edit = W.QTextEdit()
> edit.setText(C.qVersion())
> 
> button = W.QPushButton("broken")
> button.clicked.connect(edit.clear)
> 
> button2 = W.QPushButton("not broken")
> button2.clicked.connect(lambda: C.QMetaObject.invokeMethod(edit,
> "clear", C.Qt.QueuedConnection))
> 
> layout.addWidget(button)
> layout.addWidget(button2)
> layout.addWidget(edit)
> w.setLayout(layout)
> w.show()
> app.exec_()

I can reproduce the problem and I can confirm that an equivalent C++ 
version does not exhibit the problem.

A simpler way to get a working version is to change the connection type 
in the connect() call...

button.clicked.connect(edit.clear, C.Qt.QueuedConnection)

However I am currently at a loss about what is happening. I am using a 
version of PyQt that only uses "plain" Qt classes which does not allow 
Python re-implementations of C++ virtuals. This means that while the Qt 
event loop is running (ie. in the call to exec()), *no* PyQt is executed 
- yet the problem still exists. It's almost as if the very presence of 
Python is having an effect.

Any insight would be welcome...

Phil