[PyQt] Fwd: Mixin classes and PyQt4

[Phil Thompson]

On 16-02-2014 10:56 pm, Baz Walter wrote:
> On 16/02/14 13:27, Phil Thompson wrote:
>> On 13-02-2014 6:54 pm, Martin Teichmann wrote:
>>> Hi Baz, Hi Phil, Hi List,
>>>
>>>> This actually only relates to multiple-inheritance. It should 
>>>> never
>>>> be problem for true mixin classes.
>>>
>>> There youre certainly right. Its calling super() where I have my
>>> issues. Your examples did not call super(), so the MRO doesnt
>>> matter.
>>>
>>>
>>>> The one genuine issue with mixins in PyQt, is that you cannot
>>>> inherit custom signals.
>>>
>>> Then it might be interesting for you that with my patch applied, it
>>> does work. For example:
>>>
>>>
>>> 
>>> -----------------------------------------------------------------------
>>> from PyQt4.QtGui import QWidget, QLabel, QApplication
>>>  from PyQt4.QtCore import pyqtSignal, QObject
>>>
>>> class A(QObject):
>>>     a = pyqtSignal(str)
>>>
>>> class B(A, QLabel):
>>>     b = pyqtSignal(str)
>>>
>>> class Printer(QObject):
>>>     def print(self, a):
>>>         print(a)
>>>
>>> app = QApplication([])
>>>
>>> b = B()
>>> p = Printer()
>>> b.a.connect(p.print)
>>> b.b.connect(p.print)
>>> b.a.emit(a)
>>> b.b.emit(b)
>>> b.setText(foo)
>>>
>>> 
>>> ----------------------------------------------------------------------
>>>
>>> correctly works with my patch applied. It does not if you let A
>>> inherit from object.
>>
>> At the moment (having just started to think about it again) I'm 
>> against
>> your original patch as it is too different to the C++ behaviour.
>>
>> However I do like your other patch which raised an exception when
>> inheriting from more than one C++ class.
>
> On the face of it, this seems like an entirely good thing to do.
>
> However, I suppose it's only fair to point out that this will
> probably break code for at least some people, if this blog post is
> anything to go by:
>
>     http://trevorius.com/scrapbook/python/pyqt-multiple-inheritance/
>
> Of course, you could say that they got what they deserved for trying
> to do something in PyQt that just wouldn't be allowed in Qt itself.
> But whatever the rights and wrongs, there are going to be a few 
> people
> who might hope that this patch is only applied if the issue with
> custom signals can be resolved first.

That rules it out for PyQt4 - I don't anticipate there being many more 
PyQt4 releases and I'm not going to introduce potential breakages at 
this stage.

Phil