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

[Phil Thompson]

On 07/08/2019 16:54, Kyle Altendorf wrote:
> On 2019-08-07 07:13, Phil Thompson wrote:
>> On 05/08/2019 18:25, Kyle Altendorf wrote:
>>> 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.
>> 
>> It could also be that an exception is being correctly ignored, but not
>> cleared with PyErr_Clear().
> 
> Why would it be correct to ignore my code raising an exception?  It
> seems bad that my code has been seemingly silently raising these
> exceptions for a few years and I was unaware of it.  I guess it could
> be argued that it would be handy for PyQt to add an exception ignoring
> feature and let connectSlotsByName() continue with it's work despite
> Python code failing.  But this isn't the direction I would personally
> default.  Mostly unhandled exceptions should be seen I would think.
> 
>> However I can't reproduce the problem - I don't get a traceback,
>> although I'm not using exactly the same versions of Python.
> 
> Yes, this showed up with Python 3.7.4.  Since that's the latest
> version it is the only one that this can be replicated on afaik.
> Well, only released version.  3.8 betas may do the same thing, I
> haven't checked.  Python 3.7.3 (and whatever various versions I've run
> this with for the past few years) did not raise exceptions.

Should be fixed in tonight's snapshot.

Phil