[PyQt] pyqtsignal() and None value

J Barchan jnbarchan at gmail.com
Wed Sep 5 09:11:44 BST 2018

On 4 September 2018 at 18:08, Maurizio Berti <maurizio.berti at gmail.com>

> You are defining a specific signature in the signal:
> QtCore.pyqtSignal(int, str)
> this means that, despite the types you set in the method
> (which Qt doesn't know anything of also, as you didn't use the Slot
> decorator), whenever you emit a None (which is a NoneType, not int, nor
> str) Qt will try to "translate" it to the valid signature you assigned.
> I don't know exactly why the int is that a high number (always high and
> always random), but this probably makes sense for some C++ type signature,
> as it seems to me that the number is always 32bit long and, in my case,
> always negative.
> Anyway, if you really need to send None, you can use the generic "object"
> signature in the signal definition, or, in your case, just go with this,
> assuming the progress will never use negative numbers.
> def updateProgress(self,
> val: int=-1,
> text: str=''):
>     if val is >= 0:
>         self.progressBar.pb.setValue(val)
>     if text:
>         self.progressBar.label.setText(text)
> and then emit the text signal using -1 for val.
> Maurizio
> 2018-09-04 18:16 GMT+02:00 J Barchan <jnbarchan at gmail.com>:
>> PyQt5.7.  I am having trouble `emit()`ing a signal and receiving its
>> arguments correctly.  I have read
>> http://pyqt.sourceforge.net/Docs/PyQt5/signals_slots.html carefully.
>> *Declaration*:
>>     # class variable for "notifyProgress" signal, for displaying a
>> progressbar
>>     notifyProgress = QtCore.pyqtSignal(int, str)
>> *Initialisation*:
>> self.notifyProgress.connect(self.updateProgress)
>> *Slot*:
>>     def updateProgress(self, val: int, text: str):
>>         # slot for self.notifyProgress
>>         # eprpayrequestfunctions.runEpr() calls this to indicate progress
>>         # if it passes an integer it's the desired value for the
>> progressbar
>>         # if it passes a string it's the desired value for the label
>>         if val is not None:
>>             self.progressBar.pb.setValue(val)
>>         if text is not None:
>>             self.progressBar.label.setText(text)
>> *Signals*:
>> 1. notifyProgress.emit(None, "Some text")
>> 2. notifyProgress.emit(i / 2, None)
>> *Behaviour in slot*:
>> The problem is the passing of None from emit():
>> 1. val arrives in slot as 1261196128.
>> 2. text arrives in slot as '' (empty string).
>> *Questions*:
>>    - Where is this behaviour for None as an emit() parameter documented?
>>    - What is the correct/best way for handling this correctly/easily?
>> --
>> Kindest,
>> Jonathan
>> _______________________________________________
>> PyQt mailing list    PyQt at riverbankcomputing.com
>> https://www.riverbankcomputing.com/mailman/listinfo/pyqt
> --
> È difficile avere una convinzione precisa quando si parla delle ragioni
> del cuore. - "Sostiene Pereira", Antonio Tabucchi
> http://www.jidesk.net

I'm OK with the
val: int=-1, but not with the
text: str=''.  An int of -1 is happily an invalid value for me, butt the
trouble is a str of '' is perfectly legal, and will be used :(

(which Qt doesn't know anything of also, as you didn't use the Slot
> decorator),

I don't mind adding a slot decorator if that would help.  But presumably it
would not here, as you're saying my None value does not match the type
correctly anyway, right?

I did consider making it so there is only one parameter, and the type (int
or str) indicates which of the two paths to follow in the slot.  I followed
http://pyqt.sourceforge.net/Docs/PyQt5/signals_slots.html carefully where
it describes"The following code demonstrates the connection of overloaded
signals:" but I couldn't get it to work: it *always* called the
first/default overload, regardless of the parameter type ... :(

I'm now thinking: could I cut my losses and pass (and receive?) a single
parameter as an explicit QVariant (which we don't usually use from PyQt),
so that a single receiver slot can correctly see the original type ---
would that work/be advisable?

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