[PyQt] pyqtsignal() and None value
Maurizio Berti
maurizio.berti at gmail.com
Thu Sep 6 21:10:11 BST 2018
I don't know specifically how using QVariant could change the behavior: I'm
always using sip api version 2 also with Python 2 and Qt4 for everything,
as I find it's much more simpler and usable since it automatically converts
python types without using .toPyObject(). Note that, by using sip v2 for
everything, QtCore will not wrap QVariant nor QString/QStringList (the
latter is the default for Py3). You can specify the C++ signature type
using quotes, though, such as pyqtSignal(['QString', 'QVariant']), which
can be useful where you don't have full control over the slots, like in
designer plugins.
That said, as you might know, "object" is a much broader specification for
the object type, meaning that it includes any kind of new-style python
subclass ("object", indeed), including any PyQt wrapper. It can be useful
when you want to emit signals using complex parameters (such as object
references), but, in your case, QVariant should be enough.
Maurizio
2018-09-06 9:29 GMT+02:00 J Barchan <jnbarchan at gmail.com>:
>
>
> On 5 September 2018 at 18:38, Maurizio Berti <maurizio.berti at gmail.com>
> wrote:
>
>> First of all, if anyone here has access to the documentation pages of
>> PyQt, please read this (expecially the third point).
>>
>>
>> I do not know your exact user case, so it's hard to give you specific
>> suggestions, but here's what I'd do.
>>
>> 1. do not give any signature to the signal at all:
>>
>> notifyProgress = QtCore.pyqtSignal(object, object)
>>
>> This allows you to use None for both arguments, but I'm not familiar with
>> typing in methods for python 3 (to be fair, I didn't even know about it
>> until yesterday, as I mainly use Python2). I did a quick test and it seems
>> to work fine, though.
>>
>> 2. use a single parameter that will be sent as a tuple, and unpack it
>> within the method:
>>
>> notifyProgress = QtCore.pyqtSignal(object)
>> def updateProgress(self, *args):
>> val, text = args
>>
>> [...]
>>
>> self.notifyProgress.emit((None, "Some text"))
>> self.notifyProgress.emit((i / 2, None))
>>
>> 3. use an overloaded signal and two separate slots. This is something I
>> was struggling for a lot of time and (thanks to you! :-D) I finally found
>> out how it works, as it is *not* explained exhaustively in the
>> documentation. Also, I think that's what you were referring to.
>> As we know, signals can be connected "in two ways" (I'm referring to
>> new-style connections). Normally, you can go by simply connecting the slot
>> and eventually decorating that slot with the signature you are interested
>> in; decoration is usually not strictly required, as slot that are
>> processing invalid data are somewhat "ignored": I always thought it as some
>> kind of try/except mechanism, if at some point within the method something
>> wrong happens, the error is ignored but no exception is thrown; of course,
>> using a decorated slot avoids unnecessary computation for wrong signature
>> arguments.
>> Sometimes, though, it's better if not necessary to connect the slot to
>> the specific signature: self.somesignal[signature].connect(slot).
>> What the documentation forgets to explain is that the same _has_ to be
>> done when emitting custom overloaded signals if you are not using the
>> default signature (the first argument).
>>
>> #in this case, "str" is the default signature
>> notifyProgress = QtCore.pyqtSignal([str], [int])
>>
>> [...]
>> #the default signature can be omitted
>> self.notifyProgress.connect(self.updateProgressString)
>> self.notifyProgress[int].connect(self.updateProgressInt)
>> [...]
>>
>> @QtCore.pyqtSlot(str)
>> def updateProgressString(self, text):
>> [...]
>>
>> @QtCore.pyqtSlot(int)
>> def updateProgressInt(self, val):
>> [...]
>>
>> [...]
>>
>> #again, the default signature can be omitted
>> self.notifyProgress.emit("Some text")
>> #now, *THIS*!
>> self.notifyProgress[int].emit(5)
>>
>> I really think that this is something the documentation should *not*
>> miss. I understand that from the C++ point of view it's pretty obvious that
>> a signal is emitted according to its argument type signature, but this is
>> not that obvious for a common Python programmer.
>>
>> Obviously, in the last case there's a small drawback: you can't pass None
>> as argument, as it will be converted to the typed signature, giving you an
>> empty string or that infamous random 32bit integer. I don't think it would
>> be an issue in your case, but it is something to keep in mind anyway.
>>
>>
>> I hope this helps... it helped me :-)
>>
>> Maurizio
>>
>> 2018-09-05 12:33 GMT+02:00 J Barchan <jnbarchan at gmail.com>:
>>
>>>
>>>
>>> On 5 September 2018 at 09:11, J Barchan <jnbarchan at gmail.com> wrote:
>>>
>>>>
>>>>
>>>> On 4 September 2018 at 18:08, Maurizio Berti <maurizio.berti at gmail.com>
>>>> wrote:
>>>>
>>>>> 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/Do
>>>>>> cs/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?
>>>>
>>>>
>>>> --
>>>> Kindest,
>>>> Jonathan
>>>>
>>>
>>> OK, I gave QVariant (single parameter) a try. *All* I had to was:
>>>
>>> 1. Declare signal as:
>>>
>>> QtCore.pyqtSignal(QVariant)
>>>
>>> 2. Write slot like:
>>>
>>> def updateProgress(self, val):
>>> if type(val) is int or type(val) is float:
>>> self.progressBar.pb.setValue(val)
>>> elif type(val) is str:
>>> self.progressBar.label.setText(val)
>>>
>>> 3. And then it works correctly with both emitter types (without caller
>>> bothering with QVariant), like:
>>>
>>> notifyProgress.emit(50)
>>> notifyProgress.emit("Finishing...")
>>>
>>> Is there any reason I should *not* be doing this, because it seems
>>> perfect to me?
>>>
>>>
>>>
>>> --
>>> Kindest,
>>> Jonathan
>>>
>>
>>
>>
>> --
>> È difficile avere una convinzione precisa quando si parla delle ragioni
>> del cuore. - "Sostiene Pereira", Antonio Tabucchi
>> http://www.jidesk.net
>>
>
> Hi Maurizio,
>
> First, thank you for your time on this matter.
>
> I think the bit you identified which I would not have tried is the
> (non-documented) need to invoke the non-default overload via
> #now, *THIS*!
> self.notifyProgress[int].emit(5)
> But I would still welcome your comment on my last post? There I said I
> have solved the whole thing very simply, just by using
>
> QtCore.pyqtSignal(QVariant)
>
> as my signal declaration. To be clear: although I originally asked for
> two separate parameters or overloads for the string versus the int, in my
> case I am quite content with one at a time (calling code tends *either*
> to pass the int *or* the string, it doesn't need both in one call).
>
> My problem then was the type for the parameter, to cater for either str or
> int. I chose QVariant, and it works fine for me. Your suggestion seems
> to be object (I don't know what that maps to in C++). Do you think it
> makes any difference if I stick with my choice?
>
> Thanks.
>
> --
> Kindest,
> Jonathan
>
--
È difficile avere una convinzione precisa quando si parla delle ragioni del
cuore. - "Sostiene Pereira", Antonio Tabucchi
http://www.jidesk.net
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