PyQt5. Multiple inheritance and __new__

Question

I'm working on simple PyQt5 app which would allow me to edit some files like .xml, .txt

This is how it look right now:

Because I need to work with different files I decided to implement superclass which will found and return appropriate subclass depend on file extension.

class WidgetHandler:
    def __new__(cls, path, parent=None):
        ext = os.path.splitext(path)[1].replace('.', '')
        for subclass in cls.__subclasses__():
            if subclass.__name__.lower() == ext:
                return super().__new__(subclass)
        raise NotImplementedError

    def __init__(self, parent, path):
        self._path = path
        self._parent = parent
        self._is_modified = False
        self._create()

    def _create(self):
        raise NotImplementedError

    def _save(self):
        raise NotImplementedError

    def _close(self):
        raise NotImplementedError

    def save(self) -> None:
        if self._is_modified:
            self._save()
            self._is_modified = False

    def close(self) -> None:
        if self._is_modified:
            pass  # TODO Запрос сохранения
        self._close()


class XML(WidgetHandler, QTreeView):
    def _create(self):
        self.setParent(self._parent)
        model = DomModel(self._path)
        # model.dataChanged.connect(lambda: print('Data Changed'))
        self.setModel(model)
        self.expandAll()
        self.setRootIsDecorated(False)
        self.setItemsExpandable(False)
        delegate = XMLDelegate()
        self.setItemDelegate(delegate)

    def _save(self):
        pass

    def _close(self):
        pass

But instead of widget I get this exception: object.__new__(XML) is not safe, use QTreeView.__new__(). Could someone please tell me, what am I doing wrong?

Answer 1

The superclass of WidgetHandler is (implicitly) object, since you haven't defined an explicit superclass. So you are calling object.__new__(XML). I Googled your error and found that for Python 2.6,

You can't use object as the terminating class for your inheritance if you want to pass arguments to __new__ or __init__.

I don't know what version of Python you're using, but this could be the source of your problem.

I try to avoid overriding __new__ in general; for this application, you should consider using the factory pattern instead.