[vtkusers] python exceptions patch needed for vtk 5.6

David Gobbi david.gobbi at gmail.com
Sun May 30 21:03:15 EDT 2010 

Hi Florian,

Here is an example of how to make an error observer in python.  Most
of the code is straightforward.  The __call__ method is needed because
VTK-Python observers must be callable objects.  The only messy part is
the "CallDataType", which is needed because in C++ the VTK object
passes the error message as a "void *" instead of as a properly typed
object, so the python observer needs a hint that the "void *" is a
"char *".

==== python code ====
import vtk

class ErrorObserver:

    def __init__(self):
        self.__ErrorOccurred = False
        self.__ErrorMessage = None
        self.CallDataType = 'string0'

    def __call__(self, obj, event, message):
        self.__ErrorOccurred = True
        self.__ErrorMessage = message

    def ErrorOccurred(self):
        occ = self.__ErrorOccurred
        self.__ErrorOccurred = False
        return occ

    def ErrorMessage(self):
        return self.__ErrorMessage

e = ErrorObserver()

a = vtk.vtkImageReader()
a.AddObserver('ErrorEvent', e)

print "doing update"
a.Update()
if e.ErrorOccurred():
    print e.ErrorMessage()
==== python code ====



On Sun, May 30, 2010 at 3:47 AM, Florian Bruckner <e0425375 at gmail.com> wrote:
> OK, for me this means that I will use vtk observers, since I need a portable
> solution that runs on a default vtk installation.
>
> @gobbi: could you please send me a short example, of how you use these
> observers in python?
>
> nevertheless it would be nice to see this patch in some future vtk releases.
> If the only problem are these unreachable code warnings in windows, why
> can't you simply add some helper-functions that execute this otherwise
> unreachable code? Since I only use Linux I will not be able to test it for
> myself, but I can't imagine that none of the windows programmers could find
> a solution for this. perhaps you should give it another try and repost the
> patch to the developers list?
>
> thanks to all for your help
> I wish you what
> FloB
>
>
> On Sat, May 29, 2010 at 4:36 AM, David Gobbi <david.gobbi at gmail.com> wrote:
>>
>> Hi Charl,
>>
>> I've looked over the patch and I have one question.  What happens when
>> an python exception is thrown in a python observer callback?  I didn't
>> see any modifications to vtkPythonCommand, so I'm guessing that the
>> error will just be printed (like it is now) and not fed back to the
>> original caller.
>>
>>   David
>>
>>
>> On Fri, May 28, 2010 at 4:43 PM, Charl Botha <c.p.botha at tudelft.nl> wrote:
>> > Dear Florian and David,
>> >
>> > On 28 May 2010 21:07, Florian Bruckner <e0425375 at gmail.com> wrote:
>> >>
>> >> is there a reason why the patch was not merged into the official vtk
>> >> release?
>> >>
>> >> i'm just wondering because that patch is fairly old and for me this
>> >> seems to be a very good solution for a general problem.
>> >
>> > The patch has a permanent home here:
>> >
>> > http://code.google.com/p/devide/source/browse/trunk/johannes/patches/pyvtk_tryexcept_and_pyexceptions.diff
>> >
>> > Of course I think it's a good patch. :) It does the following:
>> > 1. Turns all VTK errors into Python exceptions.
>> > 2. Carefully releases the GIL during all calls into VTK (grabbing it
>> > back for observers) so that you can successfully explicitly
>> > multi-thread VTK pipelines from Python.
>> > 3. Tries to trap bad_alloc.
>> >
>> > I've been working on this for years, but due to Windows unreachable
>> > code warnings that I have not been able to solve, I've never committed
>> > this to VTK (it works, just gives many warnings during compilation).
>> > My own binaries all have this patch integrated (DeVIDE [1] and VTK
>> > [2]).
>> >
>> > The current version is for 5.4.2, I'd like to update for 5.6 as soon
>> > as I can figure out how to check out a clean 5.6 with git (until very
>> > recently there was no tag yet, I see that it's there now!
>> > http://vtk.org/gitweb?p=VTK.git;a=summary )
>> >
>> > I'm also considering to make a wiki page on devide.googlecode.com that
>> > documents the patch.
>> >
>> > See you,
>> > Charl
>> >
>> > [1] http://graphics.tudelft.nl/Projects/DeVIDE
>> > [2] http://cpbotha.net/software/latest-vtk-windows-binaries/
>> >
>> >
>> >>
>> >> On Fri, May 28, 2010 at 4:07 PM, David Gobbi <david.gobbi at gmail.com>
>> >> wrote:
>> >>>
>> >>> Hi Florian,
>> >>>
>> >>> I haven't seen mention of the patch here on the list, and I know
>> >>> that it isn't in VTK 5.6 or in VTK devel.  For my own apps, I use
>> >>> VTK observers to catch reader errors.
>> >>>
>> >>>   David
>> >>>
>> >>>
>> >>> On Fri, May 28, 2010 at 12:52 AM, Florian Bruckner
>> >>> <e0425375 at gmail.com> wrote:
>> >>> > hi,
>> >>> >
>> >>> > I just figured out the problem that no python exceptions are raised
>> >>> > if
>> >>> > my reader produces an error. I use vtk-5.6.
>> >>> >
>> >>> > after searching the net and the mailing list I found that there are
>> >>> > some
>> >>> > patches available
>> >>> > (turn-vtk-errors-into-python-exceptions-and-catch-bad-malloc patch)
>> >>> > which should solve this problem. Unfortunately I could neither find
>> >>> > such
>> >>> > a patch for vtk-5.6 nor any actual posts about this topic.
>> >>> >
>> >>> > So should this patch allready be included in the new vtk-5.6? or
>> >>> > where
>> >>> > can I get it otherwise? are there any reasons why the patch is not
>> >>> > included in the official release? or is there another work around
>> >>> > that
>> >>> > is proposed for this kind of problems?
>> >>> >
>> >>> > thanks
>> >>> > FloB
>> >>
>> >>
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>> >
>> >
>> >
>> > --
>> > dr. charl p. botha :: http://graphics.tudelft.nl/CharlBotha
>> > work: cpbotha at medvis.org personal: cpbotha at cpbotha.net
>> >
>> > See our latest papers on shape spaces, brain connectivity, shoulder
>> > segmentation, human motion analysis, voxel classification and eye fat
>> > deformation! http://graphics.tudelft.nl/Publications/2010
>> >
>
>

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