[PyQt] Re: problem with QAbstractItemModel

Linos info at linos.es
Sat Nov 7 09:23:31 GMT 2009

Vicent Mas escribió:
> On 2009-11-06 Baz Walter <bazwal at ftml.net> said:
>> ...
>> your implementation of QAbstractItemModel.index is going to get called
>> *a lot* and in many unforseen ways. 
> Yes, but it is a general fact, not something that happens to my particular 
> implementation, right?
>> i think a more sane implementation
>> would be something like this:
>>      def index(self, row, column, parent):
>>          if self.hasIndex(row, column, parent):
>>              branch = self.nodeFromIndex(parent)
>>              return self.createIndex(row, column, branch.childAtRow(row))
>>          return QModelIndex()
>> this fixes your immediate bug, and will probably avoid many others.
> I know that my implementation is very rough, but it is intended just for 
> testing purposes. Of course your workaround and other similar (like the one 
> sent by Linos) fixes the problem, but they leave the main question unanswered:  
> why wrong row values are passed to the index method? Is it expected? If it is 
> developers are forced to include good sanity code in their index method 
> implementation (as you and Linos did), but I've not seen any reference to this 
> in the Qt documentation. IMHO the Qt model implementation should not allow 
> wrong row values to be passed to the index method. In fact, the example in the 
> 'Editable Tree Model Example' section of the Qt documentation doesn't check at 
> all the validity of the row argument passed to the index method 
> implementation.
> On the other hand the fact that my script works fine with some Qt/PyQt versions 
> but fails with recent versions seems to indicate that there is a bug either in 
> the Qt implemention of models or in that PyQt versions.
> Vicent
> PS: Baz, thanks for your code snippet
> ::
> 	Share what you know, learn what you don't

	in the qt examples from qt 4.5 documentation you can see they use this in the 

QModelIndex TreeModel::index(int row, int column, const QModelIndex &parent)
      if (!hasIndex(row, column, parent))
          return QModelIndex();

      TreeItem *parentItem;

      if (!parent.isValid())
          parentItem = rootItem;
          parentItem = static_cast<TreeItem*>(parent.internalPointer());

      TreeItem *childItem = parentItem->child(row);
      if (childItem)
          return createIndex(row, column, childItem);
          return QModelIndex();

and this in the item class.

TreeItem *TreeItem::child(int row)
      return childItems.value(row);

but they can do this because QList .value method dont get an error if it is out 
of bounds.

T QList::value ( int i ) const

Returns the value at index position i in the list.

If the index i is out of bounds, the function returns a default-constructed 
value. If you are certain that the index is going to be within bounds, you can 
use at() instead, which is slightly faster.

I am using this same strategy but like i am using python list i check before if 
i can return the children, you can use a "try except" too.

Miguel Angel.

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