[PyQt] Changing the wrapped object

[Jens Thoms Toerring]

On Fri, May 04, 2012 at 01:42:11PM +0100, Phil Thompson wrote:
> On Fri, 4 May 2012 14:33:10 +0200, Jens Thoms Toerring <jt at toerring.de>
> wrote:
> >    I have a C++ class SIP-wrapped with the '/Abstract/'
> > annotation, Thus Pyton just has a pointer to instances of
> > this class (and ownership is with C++). Now, at a certaind
> > point I would like to make the Python wrapper object point
> > to a different instance (of the same class). Finding the add-
> > ress of the Python object is simple using sipGetPyObject().
> > But is there some way make that Python object refer to a dif-
> > ferent of my C++-objects? I guess that there's some pointer
> > to my C++ object in that Python object that just need to be
> > modified to point to the new instance of the C++ class. I've
> > been going through all of the dicumentation several times but
> > haven't found anything that looks as if would do something
> > like that. And, of course, I wouldn't like to mess with un-
> > documented features of the SIP-wrapper, resulting in every-
> > thing breaking with the next SIP release;-)
> 
> There is no supported way of doing this.

Thank you. Bit of a pity. Guess I'll have to find some other
way of dealing with this...
 
In case you're interested in the background here's an attempt
to explain what I'm doing. Consider you have a class for dir-
ectories and a class for files in that directory. Also assume
that for some reasons (see below) the ownership must remain
with C++. Now the directory class has a method to delete the
directory. Thus the C++ object for the directory object as
well all the objects for the files in the directory get de-
leted. No problem with C++ - I simply document that if you
have used the delete() method using the directory object and
all the file objects derived from it can't be used anymore -
C++ programmer are used to such things. But Python still has
the wrapper objects with the pointers and if the user tries
to use them afterwards the whole script will (in the best of
all cases) crash. That doesn't look very "pythonic" to me -
I guess a Python programmer will expect the script to abort
with an exception, explaining about the errors of his way.
Thus I would like to "redirect" the Python wrapper objects
to a new C++ objects that, when used, throw an exception
(and transfer ownership for them to Python, so they get
deleted when Python garbage-collects the wrapper objects).

If you wonder why I need to keep the ownership to C++ there
are two reasons (the actual stuff I do isn't about directories
and files, it's just simpler to visualize it that way): these
objects have a lot of data (several hundreds of MB) and making
copies of them simply would require too much memory. Moreover,
under some circumstanes these objects must be immutable - and
Python doesn't seem to have C++'s notion of const-ness.

                             Best regards, Jens
-- 
  \   Jens Thoms Toerring  ________      jt at toerring.de
   \_______________________________      http://toerring.de