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

[J Barchan]

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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