[PyQt] New exception in Python 3.7.4 when bad code tries to PyQt5.uic.loadUi()

Kyle Altendorf sda at fstab.net
Mon Aug 5 18:25:51 BST 2019

On 2019-08-03 14:38, Kyle Altendorf wrote:
> I have many custom widgets with pyqtProperty's and GUI elements loaded
> from .ui files.  In various ways, my existing code tries to access
> not-yet-present attributes in pyqtProperty's while the .ui is being
> loaded.  This is obviously bad and I plan to fix my code.  I share
> this example because I already finished it and because this is an odd
> change of behavior from a micro version bump of Python.  Also, it is
> odd that each first time my code raises an exception there is not
> necessarily a Qt failure.  For example, WidgetB must be constructed
> twice to get the traceback.  I understand that this may well not get
> 'fixed' since it probably never should have worked to begin with.
> Code and output from 3.7.3/3.7.4 on Linux/Windows are attached and
> also available at:
>     https://gist.github.com/altendky/f15f8d180269a4080c25dec83cf556a0
> Here is the basic traceback:
>     AttributeError: __getattribute__
>     The above exception was the direct cause of the following 
> exception:
>     Traceback (most recent call last):
>       File "bad.py", line 146, in main
>         widgets.append(WidgetA(broken=True))
>       File "bad.py", line 53, in __init__
>         PyQt5.uic.loadUi(io.StringIO(ui_text), self)
>       File
> "C:\epc\bug\venv3.7-32\lib\site-packages\PyQt5\uic\__init__.py", line
> 227, in loadUi
>         return DynamicUILoader(package).loadUi(uifile, baseinstance,
> resource_suffix)
>       File
> "C:\epc\bug\venv3.7-32\lib\site-packages\PyQt5\uic\Loader\loader.py",
> line 72, in loadUi
>         return self.parse(filename, resource_suffix, basedir)
>       File
> "C:\epc\bug\venv3.7-32\lib\site-packages\PyQt5\uic\uiparser.py", line
> 1030, in parse
>         actor(elem)
>       File
> "C:\epc\bug\venv3.7-32\lib\site-packages\PyQt5\uic\uiparser.py", line
> 942, in createConnections
>         QtCore.QMetaObject.connectSlotsByName(self.toplevelWidget)
>     SystemError: <built-in function connectSlotsByName> returned a
> result with an error set

I read briefly about this SystemError and it seems that while yes, my 
code was raising an exception and shouldn't really have worked ever...  
this error indicates that connectSlotsByName() isn't properly 
propagating the exception and is instead returning a value as if no 
exception occurred.  As such it seems that there is an error in the PyQt 
connectSlotsByName() wrapper in addition to my code.  Though I 
admittedly don't know my way around SIP nor the C-API much.


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