[PySide] Bug with new hash table feature

[Hugo Parente Lima]

On Wednesday, June 13, 2012 05:32:26 PM Nathan Smith wrote:
> We don't want to raise a RuntimeError when calling the hash function in
> Shiboken because of the very reason you stated.  Doing something that
> raises exceptions in the hash function will break any container that uses
> hashes (weak references, sets, dicts, weakref.WeakSet, probably many more).
>  Returning default error values has a similar effect.
> 
> My most recent approach was to return the address of the shiboken object,
> which is address of the PyObject.  I tried this approach (I haven't
> submitted it yet) on the example you provided below, and it seems to work.
>  I've been using it with my code for a few days now, and it's been working
> well.
> 
> Note that using the address method has the drawback that two objects
> separated in time may have the same hash value because they may land at the
> 
> same memory address.  You can see this in the following test case:
> >>> hash(QtCore.QObject()) == hash(QtCore.QObject())
> 
> True
> 
> 
> The first QObject is created, the hash is taken of it, and then it is
> destroyed.  The reference count drops to 0, so the Shiboken handle is
> cleaned up.  A second QObject is created (at the same address as the first
> QObject), the hash is taken of it, and then it is destroyed.  This could be
> avoided if we kept a running counter of SbkObject instances and used the
> counter as the hash value (which I think is overkill).
> 
> Incidentally, what is the difference between shiboken.delete and
> shiboken.invalidate?  There don't appear to be any docstrings in the
> shiboken module and invalidate isn't in the online documentation.

The docs are on the PSEP[1] and on Shiboken docs[2].

delete really deletes the underlying C++ object then invalidate the Python 
object, i.e. any use of this object will raise a exception.

invalidate only invalidates the Python object, causing it to raise an 
exception when used.

[1] http://www.pyside.org/docs/pseps/psep-0106.html
[2] http://www.pyside.org/docs/shiboken/shibokenmodule.html
 
> Nathan
> 
> On Wed, Jun 13, 2012 at 3:45 PM, John Ehresman <jpe at wingware.com> wrote:
> > On 5/30/12 3:35 PM, John Ehresman wrote:
> >> Can we use the address of the Shiboken object as the hash value?  That
> >> 
> >>> remains valid so long as there are references to the object, even after
> >>> the object itself has been deleted in C++ land.
> >> 
> >> This works if there can only be one wrapper at a time for a given
> >> QObject.  I don't know if this is the case.
> > 
> > I just ran into this bug and tried to apply the patch locally, but ran
> > into poblems.  If the address of the PyObject* can be used, I think that
> > 
> > would be preferable.  Consider the following:
> >    o = QObject()
> >    d = {}
> >    d[o] = 1
> >    
> >    def on_destroy():
> >        d.pop(o)
> >    
> >    o.destroyed.connect(on_**destroy)
> >    shiboken.delete(o)
> > 
> > The pop in the destroy handler will fail with a RuntimeError and even if
> > the RuntimeError is suppressed and a default hash value returned, the
> > entry
> > in the dictionary won't be found or removed.
> > 
> > John
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 198 bytes
Desc: This is a digitally signed message part.
URL: <http://lists.qt-project.org/pipermail/pyside/attachments/20120614/98b70412/attachment-0001.sig>