model.index() returns wrong class

nst0022

If you run either this PyQt5 Python script:

from PyQt5.QtWidgets import QApplication
from PyQt5.QtWidgets import QTreeView
from PyQt5.QtCore    import QModelIndex
from PyQt5.QtGui     import QStandardItem
from PyQt5.QtGui     import QStandardItemModel

app             = QApplication([])
treeview1       = QTreeView()
model           = QStandardItemModel()
parentModelItem = model.invisibleRootItem()
standardItem    = QStandardItem('--')

parentModelItem.appendRow((standardItem))

modelIndex      = model.index(0, 0, QModelIndex())

print(type(modelIndex.model()))
# output: <class 'PyQt5.QtGui.QStandardItemModel'>

or this Pyside2 script:

from PySide2.QtWidgets import QApplication
from PySide2.QtWidgets import QTreeView
from PySide2.QtCore    import QModelIndex
from PySide2.QtGui     import QStandardItem
from PySide2.QtGui     import QStandardItemModel

app             = QApplication([])
treeview1       = QTreeView()
model           = QStandardItemModel()
parentModelItem = model.invisibleRootItem()
standardItem    = QStandardItem('--')

parentModelItem.appendRow((standardItem))

modelIndex      = model.index(0, 0, QModelIndex())

print(type(modelIndex.model()))
# output: <class 'PySide2.QtGui.QStandardItemModel'>

the result in both cases is that the index() function returns a QStandardItemModel object.

However, documentation says, the result is a QAbstractItemModel

The difference is the function signatures:

(1) index() of QStandardItemModel requires row and column only, while
(2) index() of QAbstractItemModel requires row, column and parent index

Only (2) allows a unique access of items in a non-trivial QTreeView.

As there is no other way to access the current selected treeview item, this appears to be a show stopper.

Am I doing something wrong?

/////////////

Debian 10.10 Buster
PySide2 5.15.2
Python 3.7.3

nst0022

Got it.

However, the result differs from the claimed problem in this thread:

I have a QTreeView.currenIndex() and wanted to retrieve an item from it, then obtain different data columns from this item, which led to the problem.

This is all nonsense.

With a QModelIndex, a column can easily be retrieved with:

myCurrentIndex.sibling(row, col)

And that's it.

@JonB: I apologize for having made these waves and thank you for your patient assistance, which helped me find the solution :-).

JonB

@nst0022
Hi and welcome.

Thank you for laying out your question neatly, better than many do here :)

I can see you know what you're talking about, but I'm struggling to understand what actually goes wrong for you? The method returns a QAbstractItemModel, and the instance is a QStandardItemModel, which inherits from QAbstractItemModel, so what is wrong? I don't recognise your difference in function signatures, QAbstractItemModel.index() takes a parent index defaulting to none and that method is overridden by QStandardItemModel to do whatever extra processing but still with that third parameter. Pass your parent index whenever you have one.

nst0022

@JonB: Thanks for your quick and concise reply.

@JonB said in model.index() returns wrong class:

Pass your parent index whenever you have one.

That does not work, I already did that and extended my example above:

from PySide2.QtWidgets import QApplication
from PySide2.QtWidgets import QTreeView
from PySide2.QtCore    import QModelIndex
from PySide2.QtGui     import QStandardItem
from PySide2.QtGui     import QStandardItemModel

app              = QApplication([])
treeview1        = QTreeView()
model            = QStandardItemModel()
parentModelItem  = model.invisibleRootItem()
standardItem     = QStandardItem('--')

parentModelItem.appendRow((standardItem))

modelIndex       = model.index(0, 0, QModelIndex())

#new:
modelIndexParent = modelIndex.parent()
item             = modelIndex.model().item(0, 0, modelIndexParent)

The result is:

python3 bug_1_pyside2_b.py
Traceback (most recent call last):
  File "bug_1_pyside2_b.py", line 18, in <module>
    item             = modelIndex.model().item(0, 0, modelIndexParent)
TypeError: PySide2.QtGui.QStandardItemModel.item(): too many arguments

this error message is contrary to:

@JonB said in model.index() returns wrong class:

is overridden by QStandardItemModel ... but still with that third parameter

I guess the third parameter is inadvertently omitted in the implementation of QStandardItemModel.

JonB

@nst0022 said in model.index() returns wrong class:

I guess the third parameter is inadvertently omitted in the implementation of QStandardItemModel.

Let's at least start from the C++ documentation for Qt:

QModelIndex QAbstractItemModel::index(int row, int column, const QModelIndex &parent = QModelIndex()) const
This is marked [pure virtual], which means it has no implementation in QAbstractItemModel and must be overridden in inheritors.

QModelIndex QStandardItemModel::index(int row, int column, const QModelIndex &parent = QModelIndex()) const
This is marked [override virtual]. And documented:

Reimplements: QAbstractItemModel::index(int row, int column, const QModelIndex &parent) const.

QStandardItemModel does not offer any other definition for index().

Now I'm getting lost. You originally were asking about index() method. You now show an error message

    item             = modelIndex.model().item(0, 0, modelIndexParent)
TypeError: PySide2.QtGui.QStandardItemModel.item(): too many arguments

That is item() method.

QAbstractItemModel does not have any item() method to override, and QStandardItemModel has just item(int row, int column = 0) which is indeed 2 not 3 arguments.

Before I look any further, could you clarify which method you are finding at fault?

BTW, if you are wanting a tree model from a QStandardItemModel then https://doc.qt.io/qt-5/qstandarditemmodel.html#details gives some sample code. Note it includes:

Since a QAbstractItemView provides QModelIndex-based signals and functions, you need a way to obtain the QStandardItem that corresponds to a given QModelIndex, and vice versa. itemFromIndex() and indexFromItem() provide this mapping.
QStandardItem *item = myStandardItemModel->itemFromIndex(index);

nst0022

@JonB You are right, I confused item() and index()

Now I am totally at lost.
I have to thoroughly revisit the subject.

JonB

@nst0022
Indeed :)

I think that last "BTW" of mine might be what you are looking for in a hierarchical tree model: QStandardItem *QStandardItemModel::itemFromIndex(const QModelIndex &index) const.

I am unsure how QStandardItem *QStandardItemModel::item(int row, int column = 0) const treats row in a hierarchical model, you would have to play with that to compare against itemFromIndex() which accepts a QModelIndex.

nst0022

Got it.

However, the result differs from the claimed problem in this thread:

I have a QTreeView.currenIndex() and wanted to retrieve an item from it, then obtain different data columns from this item, which led to the problem.

This is all nonsense.

With a QModelIndex, a column can easily be retrieved with:

myCurrentIndex.sibling(row, col)

And that's it.

@JonB: I apologize for having made these waves and thank you for your patient assistance, which helped me find the solution :-).

JonB

@nst0022
Yes, QModelIndex.sibling() is a convenient way to access another item from a QModelIndex. For same row but different column QModelIndex QModelIndex::siblingAtColumn(int column) const is even more convenient.

nst0022

I have now figured out, why I had trouble with the item() and index() functions.

If you look-up code on the Internet, both C and Python examples exist, where a model has an index() function.

However, the Python documentation of QStandardItemModel does not mention index() in its function list, where the corresponding C documentation does (in Reimplemented Public Functions), but the function can be used in Python anyway.

I assume, that it does not make sense to report this kind of Python documentation errors, as long as the examples still contain C code.

JonB

@nst0022
When I first did Qt I was Python. That was PyQt5. PySide2 didn't exist yet, and that documentation you refer to is for PySide2. There isn't an equivalent for PyQt5. The PySide2 stuff largely holds.

So PyQt5 made you look up the C++ Qt documentation anyway. My advice would be you should combine that C++ look up with the PySide2. The PySide2 is mostly some kind of "copy" from the C++. It may have omissions. Anyway, there's my tip, use the C++ as well at least.

nst0022

@JonB Aha, good to know.

One should never be too picky with documentation :-).