[PySide] Is dual inheritance clearer?

[Nik Melchior]

On Thu, Apr 04, 2013 at 08:10:29AM +0300, anatoly techtonik wrote:
> On Thu, Apr 4, 2013 at 8:02 AM, Algis Kabaila <akabaila at pcug.org.au> wrote:
>
> > **
> >
> > Hi,
> >
> >
> >
> > I believe that PySide (and PyQt for that matter) are widely used for GUI
> > design. There are two main "orderly" ways to start a GUI program, after
> > designing it in the Qt Creator:
> >
> >
> >
> > (1)
> >
> > With dual inheritance:
> >
> > class MainWindow(QMainWindow, Ui_MainWindow):
> >
> > def __init__(self, parent=None):
> >
> > super(MainWindow, self).__init__(parent)
> >
> > self.setupUi(self)
> >
> >
> >
> > (2)
> >
> > Without dual inheritance:
> >
> > class MainWindow(QMainWindow):
> >
> > def __init__(self, parent=None):
> >
> > super(MainWindow, self).__init__(parent)
> >
> > self.ui = Ui_MainWindow
> >
> > self.ui.setupUi(Ui_MainWindow(),self)
> >
> >
> >
> > There have been some horror stories about the evils of multiple
> > inheritance. What is the preferred method, (1) or (2)?
> >
>
> I'd say - that depends on what Ui_MainWindow is, but in general case (2) is
> better, because:
>
>  * self.ui defines a namespace to look for methods of UiMW object
>  * explicit is better than implicit

I'm sorry this doesn't help resolve your confusion, but I prefer the opposite
approach, due to:

  * Consistency with hand-written UI:  When I write simple applications or
    widgets that compose other widgets, the contained widgets are merely
    members; I do not relegate them to a separate namespace.  It's quite
    simple to do most layouts writing the code by hand, without designer.
  * IS-A vs. HAS-A: Your MainWindow class IS-A QMainWindow as much as it IS-A
    UI_MainWindow.  It cannot be instantiated without both parent classes, and
    it does not make sense to consider replacing the self.ui object with
    anything polymorphicly related to the Ui_MainWindow class.  I think the
    only reason the UI_MainWindow is a separate class is due to the
    convenience of placing auto-generated code into its own file.

It's really little more than a style choice, but I think the consistency
argument will be important as you start to write more and more Qt/PySide
classes.  Unless your hand-coded UIs contain a self.ui namespace, you will
confuse yourself trying to remember if a particular QObject's member functions
should access its widgets as, say, self.labelWidget or self.ui.labelWidget.

--
Nik Melchior
Sr. Roboticist       | 412-379-4500 x147
Seegrid Corporation  | www.seegrid.com
216 Park West Drive, Pittsburgh, PA 15275
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