[PyQt] Slot connections fail with newest snapshot

[Phil Thompson]

On 21 Jul 2016, at 4:08 pm, Florian Bruhin <me at the-compiler.org> wrote:
> 
> * Phil Thompson <phil at riverbankcomputing.com> [2016-07-21 15:27:13 +0100]:
>> On 21 Jul 2016, at 3:05 pm, Florian Bruhin <me at the-compiler.org> wrote:
>>> 
>>> * Phil Thompson <phil at riverbankcomputing.com> [2016-07-21 15:03:21 +0100]:
>>>> On 21 Jul 2016, at 1:51 pm, Florian Bruhin <me at the-compiler.org> wrote:
>>>>> 
>>>>> * Phil Thompson <phil at riverbankcomputing.com> [2016-07-17 12:57:24 +0100]:
>>>>>>>> The current behaviour is different, but what are you expecting to
>>>>>>>> happen (given your class does not inherit from QObject)?
>>>>>>> 
>>>>>>> Oh, @pyqtSlot only works on classes inheriting from QObject? While
>>>>>>> that makes some sense, that's news for me!
>>>>>>> 
>>>>>>> I think either it should just be ignored (like before?), or show some
>>>>>>> useful error message like "pyqtSlot can only be used on classes
>>>>>>> inheriting from QObject" or so. I also think this should be pointed
>>>>>>> out in the docs for @pyqtSlot - while the examples do inherit from
>>>>>>> QObject, it's not explicitly documented anywhere I think.
>>>>>> 
>>>>>> You can use it in classes that are used as mixins with QObject derived classes.
>>>>> 
>>>>> Hmm, I see.
>>>>> 
>>>>> So for normal classes which are not QObject subclasses, @pyqtSlot was
>>>>> simply ignored so far?
>>>> 
>>>> Yes.
>>>> 
>>>>> What about using @pyqtSlot on free functions (rather than methods)?
>>>>> It seems that still works fine with the current snapshot - is that
>>>>> ignored as well?
>>>> 
>>>> Yes.
>>> 
>>> Will this stay that way, or will there be a warning/error added for
>>> that as well?
>> 
>> The only error is when you try and make an invalid connection. PyQt can't tell now a decorated function or method is going to be used so it shouldn't assume somebody hasn't found a valid use case.
> 
> I still don't quite follow, sorry - what's invalid about the
> connection in my original example[1] other than I'm using @pyqtSlot on
> a non-QObject?

The main purpose of pyqtSlot is to define a new entry in the QObject's QMetaObject, so you would normally only use it on a method. pyqtSlot doesn't check that the containing class is a QObject because it might be being used as a mixin. When the connection is made the receiver must be a QObject. In your case it isn't so the connection fails.

> And how is that different to connecting to a function with @pyqtSlot,
> i.e. why is that still "valid" while the above is invalid with
> PyQt 5.7?

I agree it is inconsistent. I could add an extra check to raise an exception if connecting to a slot which is a function (as opposed to a method) decorated with pyqtSlot, but (as the decorator is completely ignored) I don't really see the point.

Phil