*** GMX Spamverdacht *** Re: [Eric] Bugs in latest eric4 snapshot

Andreas Pakulat apaku at gmx.de
Thu Apr 5 12:10:55 BST 2007

On 05.04.07 12:39:16, Jürgen Urner wrote:
> Andreas Pakulat schrieb:
> >On 05.04.07 00:03:18, Jürgen Urner wrote:
> >  
> >>Andreas Pakulat schrieb:
> >>    
> >>>Model indices are not persistent across application runs, but the Qt
> >>>views do cache model indices that they retrieve from the model. As I
> >>>said above, if you use internalPointer in python you need to make sure
> >>>to keep a reference to the object yourself, i.e. in a list or dict.
> >>>Thats why I tend to use internalId and keep a dict of id->object in the
> >>>model.
> >Anyway, I don't quite understand what you wanted to point at? The
> >problematic code in eric4 does something like
> >
> >index = sourceModel.index(row,col)
> >item = sourceModel.item(index)
> >
> >which is defined to be perfectly ok, i.e. the model can expect to get a
> >proper index with a proper internalPointer or internalId, depending on
> >which createIndex() overload it used. If an index turns out to be not
> >invalid _and_ not have a proper id/pointer then the caller is doing
> >something wrong. In the eric4 case I'm not sure yet, wether its the view
> >or the filter model, because I haven't looked at the rest of the code
> >yet...
> >
> In BrowserModel  a mapping is maintaned index.internalId --> BrowserItem.


> In cretaeIndex() a reference to the parent item is attatched to an
> index and later retrieved in calls to self.item_dict[index.internalId()]

And here's at least 1 error. eric calls createIndex with the actual
item, which means the internalPointer will return this item, however
internalId will return some totally random value, which can't be used
for indexing into the dict.

> The (Python) id or an item is mapped to items and, if I understand it right,
> the item is retrieved later by its (Qt | Python ?) id.

The model as it is available in the snapshot creates an index from an
item+row+col, but later on tries to retrieve the id from the index. The
pointer and Id don't have any connection between them, so one must be
consistent by either using the id or the pointer but never mix them.

> client calls model.index(row,col)
>    atatch pointer to parent python object to
>    index and return index

Its not the parent, but the current item for which the index is created
normally. If you do that you can use internalPointer.

> Qt calls model.parent() and maybe queries other indices
>    do something with pointer(s)

No, Qt doesn't do anything with the internal pointers, or do you mean
inside parent()? If so, yes the index that is given to parent will have
its internalPointer set properly when you did the above thing, i.e.

> client calls model.item(index)
>    do something with pointer

Right, and the client gets the index right before this call, so the
index' internalPointer or Id must be valid. If its not something is very

> Somewhere in between these calls the pointer may get lost
> and you may access an arbitrary memory location or get an index error.

No, as long as the object that you gave to createIndex is still
referenced somewhere in your application the internalPointer will still
be the valid object. Thats why you need a list or dict of objects you
already created (or create a second tree with those objects like the
simpletreemodel in the PyQt4 examples). As I said earlier what the Qt
views do internally doesn't concern us here, we can assume they do
something along this if they need an index:

index = model.index(row,col,parent)
data = model.data(index, role)

So the model can assume that every QModelIndex it gets was just created
a few milli or microseconds ago by a call to index(). If the view wants
to do caching, it needs to take special care to invalidate indexes in
the model that are no longer valid. This is possible by connecting to
the various signals a model sends out (rowsAboutToBeRemoved,
layoutChanged, reset and so on). And thats also the reason why a model
always has to call beginXXXRows/endXXXRows, to notify the view and

> I guess all comes down to the interpretation of the word "emidiate" in
> "indices are intended for emidiate use". I may be wrong, but that's how I
> figured it out for myself.

See above, generally you're assumed to retrieve an index first via the
index() function and then do your computations with it. However it is
possible to cache indices as long as care is taken that you forget about
cached indices once the model has changed.


You had some happiness once, but your parents moved away, and you had to
leave it behind.

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