[vtkusers] Status of VTK Python 3 wrapper support

[Arnd Baecker]

On Thu, 31 Jul 2014, David Gobbi wrote:

> On Thu, Jul 31, 2014 at 6:44 AM, diego0020 <da.angulo39 at uniandes.edu.co> wrote:
>>
>> But I am also looking forward to this. Is there anything we could do to push
>> support for python 3 forward?
>
> There has been some discussion about converting all of the python
> tests and examples, which is a part of the process where extra
> manpower would be a big asset.  I imagine that the conversion will
> go something like this:
>
> 1) Someone will run 2to3 on all the python code, and then make
> sure that it can be byte-compiled with python3 (it will be too early
> at this stage to actually run the code, but making sure that it
> byte-compiles will be a good first step).
>
> 2) Lots of people will then pick through the code to fix conversion
> errors and to make sure that the code actually looks like good
> python3 code.
>
> 3) Finally, someone will add some CMake magic to VTK so that if VTK
> is built against python2, cmake will run 3to2 on all of these new python3
> files and put the equivalent python2 code into the VTK build
> directory.  This is the magic that is needed to make sure that VTK
> will be able to support both python2 and python3.
>
> So any assistance with the above process would be a big help.  In
> particular, it would be very useful if someone could verify that item
> (3) is even feasible: in the end it might be necessary to have hand-
> written python2 and python3 exist side-by-side instead of using
> automatic conversion.  It _should_ be feasible, since the VTK
> tests used automated tcl-to-python conversion for many years,
> and python3 to python2 should be much easier than tcl to python.

As a disclaimer ahead: I don't have any experience in porting
to python 3. For my own projects I am just trying to follow pylint's
suggestions about possible python 3 problems and using
``from __future__ import (division, print_function)`` at the beginning.

According to https://docs.python.org/2/howto/pyporting.html
it might be worth a try to run the examples/tests with the ``-3``
option to find out about possible compatibility problems.
I tried this with some random examples and no problems were
reported.
So it might be that a lot of code is such
that it would work fine both with python 2 and python 3.
(Maybe someone from the numpy/scipy/... experts could
also help with their experience in supporting both python 2 and 3).

Unfortunately at the moment I won't have time to look into this
further, but I will try to help, if needed.

Best, Arnd