[PyQt] How to add an argument to derived class's constructor

[Phil Thompson]

On 18 Feb 2019, at 4:51 pm, J Barchan <jnbarchan at gmail.com> wrote:
> 
> 
> 
> On Thu, 14 Feb 2019 at 17:48, J Barchan <jnbarchan at gmail.com> wrote:
> 
> 
> On Thu, 14 Feb 2019 at 14:56, Vincent Vande Vyvre <vincent.vande.vyvre at telenet.be> wrote:
> Le 14/02/19 à 14:56, J Barchan a écrit :
>> 
>> 
>> On Thu, 14 Feb 2019 at 13:34, J Barchan <jnbarchan at gmail.com> wrote:
>> This may be as much a Python question as a PyQt one.  I come from a C++ background.  I do not understand the syntax/code I need in a class I am deriving from a PyQt class to allow a new parameter to be passed to the constructor.
>> 
>> I see that I asked this question a long time ago at https://stackoverflow.com/questions/45999732/python3-typing-overload-and-parameters but never got an answer.
>> 
>> I now want to sub-class from QListWidgetItem.  That starts with these constructors:
>> 
>> QListWidgetItem(QListWidget *parent = nullptr, int type = Type)
>> QListWidgetItem(const QString &text, QListWidget *parent = nullptr, int type = Type)
>> QListWidgetItem(const QIcon &icon, const QString &text, QListWidget *parent = nullptr, int type = Type)
>> QListWidgetItem(const QListWidgetItem &other)
>> 
>> My sub-class should still support these constructors.  In addition to the existing text, I want my sub-class to be able to store a new optional value.                       At minimum/sufficient I want a new possible constructor like one of the following:
>> 
>> MyListWidgetItem(const QString &text, const QVariant &value, QListWidget *parent = nullptr, int type = Type)
>> # or
>> MyListWidgetItem(const QString &text, QVariant value = QVariant(), QListWidget *parent = nullptr, int type = Type)
>> 
>> So for Python I know I start with a typing overload definition (for my editor) like 
>> 
>> @typing.overload
>> def MyListWidgetItem(self, text: str, value: typing.Any, parent: QListWidget=None, type: int=Type)
>>     pass
>> 
>> Then I get to the definition bit.  To cater for everything am I supposed to do:
>> 
>> def __init__(self, *__args)
>>     # Now what??
>>     super().__init__(__args)
>> 
>> Is that how we do it?  Is it then my responsibility to look at __args[1] to see if it's my value argument?  And remove it from __args before passing it onto super().__init__(__args)?
>> 
>> Or, am I not supposed to deal with __args, and instead have some definition with all possible parameters explicitly and deal with them like that?
>> 
>> Or what?  This is pretty fundamental to sub-classing to add parameters where you don't own the code of what you're deriving from.  It's easy in C-type languages; I'm finding it real to hard to understand what I can/can't/am supposed to do for this, I'd be really gratefully for a couple of lines to show me, please...! :)
>> 
>> -- 
>> Kindest,
>> Jonathan
>> 
>> P.S.
>> I think I got my overload a bit mixed up.  I meant I (think I) will have:
>> 
>> class MyListWidgetItem(QListWidgetItem)
>>     @typing.overload
>>     def __init__(self, text: str, value: typing.Any, parent: QListWidget=None, type: int=Type)
>>         pass
>> 
>>     def __init__(self, *__args)
>>         # Now what??
>>         super().__init__(__args)
>> 
>> 
>> -- 
>> Kindest,
>> Jonathan
>> 
>> 
>> _______________________________________________
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>> PyQt at riverbankcomputing.com
>> https://www.riverbankcomputing.com/mailman/listinfo/pyqt
> Hi,
> 
> I use just that:
> 
> class ListItem(QListWidgetItem):
>     def __init__(self, img, text, parent=None):
>         super().__init__(parent)
>         icon = QIcon()
>         icon.addPixmap(QPixmap(img), QIcon.Normal, QIcon.Off)
>         self.setIcon(icon)
>         self.setText(text)
> 
> The arguments are examples, not mandatory.
> 
> Vincent
> 
> _______________________________________________
> PyQt mailing list    PyQt at riverbankcomputing.com
> https://www.riverbankcomputing.com/mailman/listinfo/pyqt
> 
> Hello Vincent,
> 
> Thank you for replying.  I'm afraid the suggestion you give is so significantly different from what I am asking that I don't see how it addresses it.  I could write a lot about the differences between what you show and what I am asking.  Here are some:
> 	• I don't see that you're adding any argument that QListWidgetItem does not already accept??  Please bear in mind there is already QListWidgetItem(const QIcon &icon, const QString &text, QListWidget *parent = nullptr, int type = Type).  I want to add a new argument.
> 	• Your super().__init__() is not passing anything other than parent to the base constructor, yet the base constructor accepts more arguments than that, just like your derived class does.
> 	• (untested) it seems to me that your code will break the existing QListWidgetItem(const QIcon &icon, const QString &text, QListWidget *parent = nullptr, int type = Type) constructor overload (i.e. what happens when I pass a QIcon() as the img parameter to your ListItem constructor?)
> 	• how does your code allow for the existing QListWidgetItem(const QString &text, QListWidget *parent = nullptr, int type = Type) overload
> 	• you are not using any typing hints so this will lose my editor's context completion
> I don't know whether one of us is misunderstanding the other, or we're on different planes? :)
> 
> -- 
> Kindest,
> Jonathan
> 
> May I politely try bumping this question?  I have had one answer, which as far as I understand does not work.
> 
> How exactly could/would you derive from QListWidgetItem from Python to add a "value" parameter in a constructor?   I could do it easily from C++.  It does seem to me this is at least partly a PyQt question, I've tried asking it at https://stackoverflow.com/questions/54746309/python-3-add-argument-when-subclassing-from-complex-arguments , I'm getting comments like "If PyQT makes it impossible to use common Python idioms then I can't do much about it actually <g>" and "The OP needs to have a re-think and accept that some compromises are inevitable when trying to fake c++ idioms in pure python."
> 
> This is a bit above my head.  I'm either getting a generic answer which is inadequate or no answer or told it may not be doable.  I don't understand this.  Can someone tell me how to add the argument I have in mind, or something similar (you do need to read through the precise example I am asking about), or explain why it can't be done, or something?  I should be so obliged!

I know it's not what you are asking but things get much easier if you require the value to be specified as a keyword argument...

    def __init__(self, *args, value, **kwargs):
        super().__init__(*args, **kwargs)
        print("Got value", value)

...and it makes your code more readable.

Phil