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

[J Barchan]

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
>
> _______________________________________________
> PyQt mailing list    PyQt at riverbankcomputing.comhttps://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
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