# How to wrap VTK-derived C++ function for Python

**URL:** https://discourse.vtk.org/t/how-to-wrap-vtk-derived-c-function-for-python/4862
**Category:** Support
**Created:** [December 20, 2020, 1:46am UTC](https://discourse.vtk.org/t/how-to-wrap-vtk-derived-c-function-for-python/4862 "2020-12-20T01:46:50Z")
**Posts on this page:** 5
**Page:** 1

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### Author: ![desheng](https://discourse.vtk.org/user_avatar/discourse.vtk.org/desheng/32/2764_2.png) [@desheng](https://discourse.vtk.org/u/desheng)
#### Post date: [December 20, 2020, 1:46am UTC](https://discourse.vtk.org/t/how-to-wrap-vtk-derived-c-function-for-python/4862/1 "2020-12-20T01:46:50Z")

</div>

Is there any place I can find a working example to wrap a C++ function (using VTK) to be used by Python?

Searched and found this page [mix VTK and SWIG Python](https://stackoverflow.com/questions/8898041/mix-vtk-and-swig-python) but there is no compiling details, and this simple example on this page [https://public.kitware.com/pipermail/vtkusers/2002-October/013980.html](https://public.kitware.com/pipermail/vtkusers/2002-October/013980.html)

But for my system Ubuntu 16.04, vtk 8.2.0, Python 3.6.8, I could not make the example work. I have to modify them to let the compiling done without error.

Details: #============simple.cpp===================

```auto
// simple.c
//based on https://public.kitware.com/pipermail/vtkusers/2002-October/013980.html
#include "Python.h"
#include "vtkPythonUtil.h"
#include "vtkObject.h"
#include "vtkObjectBase.h"
#include <stdio.h>
#include "vtkRenderer.h"

extern "C" 
{
   static PyObject* simple_vtk_function(PyObject *self, PyObject *args);
}

static PyObject* simple_vtk_function(PyObject *self, PyObject *args)
{
   PyObject *vtkpy;
   if (!PyArg_ParseTuple(args, "O", &vtkpy))
     return NULL;
   
   vtkObjectBase *obj = vtkPythonUtil::GetPointerFromObject(vtkpy,"PyObject");
   /* Do what you want to do here. */
   //printf("%s\n", obj->GetClassName());
   
   printf("inside module simple_vtk, function simple_vtk_function\n");
   
   Py_RETURN_NONE;
}
static char simple_docs[] =
   "simple( ): Any message you want to put here!!\n";

static PyMethodDef module_methods[] = 
{
    {
      "simple_vtk_function", (PyCFunction) simple_vtk_function, 
     METH_VARARGS, simple_docs
    },
    {NULL}
};

static struct PyModuleDef simple_vtk =
  {
    PyModuleDef_HEAD_INIT,
    "simple_vtk",/* name of module */
    "",/* module documentation, may be NULL */
    -1,
    module_methods
  };

PyMODINIT_FUNC PyInit_simple_vtk(void)
{
    return PyModule_Create(&simple_vtk);
}

```

#============setup.py===================

```auto
#!/usr/bin/env python3
import distutils.core as dc
module = dc.Extension('simple_vtk',
                      sources = ['simple.cpp'],
                      include_dirs=['/usr/local/include/vtk-8.2'],
                      libraries_dirs = ['/usr/local/include/vtk-8.2'],
                      #libraries=['vtkPythonUtil']
)
dc.setup(name = 'simple_vtk',
         version = '1.0',
         ext_modules = [module])

```

compile command

CFLAGS="-std=c++11" python setup.py build

and get this printing out

```auto
/home/jun/anaconda3/lib/python3.6/distutils/extension.py:131: UserWarning: Unknown Extension options: 'libraries_dirs'
  warnings.warn(msg)
running build
running build_ext
building 'simple_vtk' extension
gcc -pthread -B /home/jun/anaconda3/compiler_compat -Wl,--sysroot=/ -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -std=c++11 -fPIC -I/usr/local/include/vtk-8.2 -I/home/jun/anaconda3/include/python3.6m -c simple.cpp -o build/temp.linux-x86_64-3.6/simple.o
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
simple.cpp: In function ‘PyObject* simple_vtk_function(PyObject*, PyObject*)’:
simple.cpp:22:19: warning: unused variable ‘obj’ [-Wunused-variable]
    vtkObjectBase *obj = vtkPythonUtil::GetPointerFromObject(vtkpy,"PyObject");
                   ^
g++ -pthread -shared -B /home/jun/anaconda3/compiler_compat -L/home/jun/anaconda3/lib -Wl,-rpath=/home/jun/anaconda3/lib -Wl,--no-as-needed -Wl,--sysroot=/ -std=c++11 build/temp.linux-x86_64-3.6/simple.o -o build/lib.linux-x86_64-3.6/simple_vtk.cpython-36m-x86_64-linux-gnu.so

```

to test in python

```auto
>>> import simple_vtk
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: /home/jun/project/play/swig/for_vtk_v2/simple_vtk.cpython-36m-x86_64-linux-gnu.so: undefined symbol: _ZN13vtkPythonUtil20GetPointerFromObjectEP7_objectPKc

```

Always got error

```auto
undefined symbol: _ZN13vtkPythonUtil20GetPointerFromObjectEP7_objectPKc

```

The C++ function is so simple, it just to convert a PyObject and convert to vtkObjectBase. I didn’t even remove the simple print printf("%s\n", obj-\>GetClassName());

But I could never make it work. I think it is a common need for people to wrap a custom C++ vtk function for Python, a little bit surpring there is almost no woring example on internet. Could you please help me on this? Thanks!

---

<div class="post-metadata">

### Author: ![Joachim\_P](https://discourse.vtk.org/user_avatar/discourse.vtk.org/joachim_p/32/3272_2.png) [@Joachim\_P](https://discourse.vtk.org/u/Joachim_P)
#### Post date: [December 20, 2020, 11:20am UTC](https://discourse.vtk.org/t/how-to-wrap-vtk-derived-c-function-for-python/4862/2 "2020-12-20T11:20:09Z")

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What do you mean by “a custom C++ vtk function”? VTK is mainly a library of C++ classes and those classes are wrapped in Python using a builtin mechanism (not SWIG) which is driven by a set of CMake macros. If you develop some new VTK-derived classes, you should be able to easily wrap them as well if your classes are organised as a custom VTK module. Does it make sense to you?

---

<div class="post-metadata">

### Author: ![desheng](https://discourse.vtk.org/user_avatar/discourse.vtk.org/desheng/32/2764_2.png) [@desheng](https://discourse.vtk.org/u/desheng)
#### Post date: [December 20, 2020, 1:30pm UTC](https://discourse.vtk.org/t/how-to-wrap-vtk-derived-c-function-for-python/4862/3 "2020-12-20T13:30:11Z")

</div>

Thanks for your reply and explanation.  
Have learned VTK has its own wrap macro also from here

> [@explain the vtk architecture design](https://discourse.vtk.org/t/explain-the-vtk-architecture-design/1978/14):
>
> Tcl/Tk bindings no longer get built (they were removed in 2017). The other two are built on some programs VTK builds that read the VTK headers and write out code for Python and Java to use VTK objects. The generated sources are then tossed back at CMake to write out the rules for actually making the libraries and modules necessary. The tools themselves are pretty tailor-made to (or at least have sizeable exceptions and special cases for) VTK classes and its class hierarchy, so they’re not genera…

What I described was not clear. Yes I mean, how to wrap a new VTK-derived classes? Is there a simplest example available?

---

<div class="post-metadata">

### Author: ![desheng](https://discourse.vtk.org/user_avatar/discourse.vtk.org/desheng/32/2764_2.png) [@desheng](https://discourse.vtk.org/u/desheng)
#### Post date: [December 20, 2020, 6:55pm UTC](https://discourse.vtk.org/t/how-to-wrap-vtk-derived-c-function-for-python/4862/4 "2020-12-20T18:55:22Z")

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Could you please give some indication which CMake file to look at for this topic, in VTK downloaed package? Is it CMake/vtkWrapPython.cmake ? Thanks.

---

<div class="post-metadata">

### Author: ![ben.boeckel](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.vtk.org/u/ben.boeckel)
#### Post date: [March 22, 2021, 12:25am UTC](https://discourse.vtk.org/t/how-to-wrap-vtk-derived-c-function-for-python/4862/5 "2021-03-22T00:25:17Z")

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In the latest release, there is code in `CMake/vtkModule.cmake` (basic wrapping infrastructure) and `CMake/vtkModuleWrapPython.cmake` for Python-specific bits. Note that the VTK-provided APIs assume you’re using the CMake infrastucture provided by VTK as well.