[vtkusers] [Python+vtk] Attaching user data to a vtkobject

[David Gobbi]

On Wed, 24 Jan 2001, Eric Boix wrote:

>         Dear vtk+Python users,
> 
> When using Python I try the following :
>    QuadActor = vtkActor()
>    QuadActor.myuserdata = "junk"
> I get the following message (on the last above line) :
>    TypeError: object has read-only attributes
> 
> This is probably due to the fact that the Python type of
> QuadActor is <type 'vtkobject'> (as opposed to <type 'instance'>)
> and hence has NO __dict__ attribute.
> 
> What is then the natural way of attaching some user data to a 'vtkobject'
> at the Python level ?
> (I'm seeking a mecanism close to the Tcl/Tk bindtags and I naively thought
> that Python dynamic typing was a straightforward way of doing that...)

Hi Eric,

Right now the reason that you can't attach user data to a vtk-python 
object is that there is no way to attach user data to the underlying
vtk-c++ object.  Here is a not-so-straightforward explanation:

The way the python wrappers work, if all python references to a vtk-python
object are deleted, then the vtk-python object is deleted (and all
your attributes would be lost).  If VTK still has references to the
vtk-c++ object, then the vtk-c++ object would still be around, and the
next time you access it through python (e.g. through a GetActors() call)
the attributes would not show up.

If you want to store user attributes for vtk-python objects, probably the
best way to do it for now is to have a global dictionary that uses the
python objects as keys.

Personally, I think it would be very nice if vtk-c++ objects could store
user attributes, but until that happens, I think that it is best if
vtk-python objects don't support user attributes either.

 - David

--
  David Gobbi, MSc                    dgobbi at irus.rri.on.ca
  Advanced Imaging Research Group
  Robarts Research Institute, University of Western Ontario