[PyQt] How to add an argument to derived class's constructor
Phil Thompson
phil at riverbankcomputing.com
Mon Feb 18 22:23:40 GMT 2019
On 18 Feb 2019, at 5:58 pm, J Barchan <jnbarchan at gmail.com> wrote:
>
>
>
> On Mon, 18 Feb 2019 at 17:27, Phil Thompson <phil at riverbankcomputing.com> wrote:
> 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
> >>
> >>
> >> _______________________________________________
> >> PyQt mailing list
> >> 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
>
> Thank you for responding. Ahh, maybe we're getting somewhere...! :)
>
> So, you're suggesting I forget about positional and add an optional value=... keyword argument? So instead of calling it like:
> addItem(MyListWidgetItem(text, value))
> caller will go:
> addItem(MyListWidgetItem(text, value=value))
> I guess that's not too bad. I'm not used to this at all, it's not how I'd do it in C++!
>
> So I'm looking at your definition closely:
>
> def __init__(self, *args, value, **kwargs):
> super().__init__(*args, **kwargs)
> print("Got value", value)
>
> So putting that value after the *args and before the **kwargs makes it so it is a specifically-named argument, is that what is going on? I was thinking I'd have to search/index/remove from the **kwargs to find my named parameter, this seems nicer....
>
> So this is how you would do it (simplest/best) if you were me and wanted to add this value parameter?
Yes - and you can make it optional as well by giving it a default value.
Phil
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