# vtkDataObject subclassing example \[python\]
**URL:** https://discourse.vtk.org/t/vtkdataobject-subclassing-example-python/7619
**Category:** Support
**Created:** [January 12, 2022, 12:57am UTC](https://discourse.vtk.org/t/vtkdataobject-subclassing-example-python/7619 "2022-01-12T00:57:32Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Nicholas\_Yue](https://discourse.vtk.org/u/Nicholas_Yue)
#### Post date: [January 12, 2022, 12:57am UTC](https://discourse.vtk.org/t/vtkdataobject-subclassing-example-python/7619/1 "2022-01-12T00:57:32Z")
I would like to pass customised data types into and out of a vtkAlgorithm subclass
Where can I find more python information about subclass vtkDataObject
The following is the gist of my idea of what I am trying to acheive
```auto
class MyCommandData(vtk.vtkDataObject):
def __init__ (self):
super(MyCommandData, self). __init__ (self)
self.stdin = ''
self.stdout = ''
self.stderr = ''
self.retcode = 0
class MyCommand(vtk.vtkAlgorithm):
def __init__ (self):
super(MyCommand, self). __init__ (self,
nInputPorts=1, inputType='MyCommandData',
nOutputPorts=1, outputType='MyCommandData')
def FillInputPortInformation(self, port, info):
if port == 0:
info.Set(vtk.vtkAlgorithm.INPUT_IS_OPTIONAL(), 1)
return 1
```
---
### Author:  [@Nicholas\_Yue](https://discourse.vtk.org/u/Nicholas_Yue)
#### Post date: [January 12, 2022, 1:38am UTC](https://discourse.vtk.org/t/vtkdataobject-subclassing-example-python/7619/2 "2022-01-12T01:38:13Z")
I have make some progress with the following but it thinks my custom data is not define
```auto
import vtk
from vtkmodules.util.vtkAlgorithm import VTKPythonAlgorithmBase
class MyCommandData(vtk.vtkDataObject):
def __init__ (self):
super(MyCommandData, self). __init__ ()
self.stdin = ''
self.stdout = ''
self.stderr = ''
self.retcode = 0
class MyCommand(VTKPythonAlgorithmBase):
def __init__ (self):
super(MyCommand, self). __init__ (nInputPorts=1, inputType='MyCommandData',
nOutputPorts=1, outputType='MyCommandData')
def FillInputPortInformation(self, port, info):
if port == 0:
info.Set(vtk.vtkAlgorithm.INPUT_IS_OPTIONAL(), 1)
info.Set(vtk.vtkDataObject.DATA_TYPE_NAME(),'MyCommandData')
return 1
def FillOutputPortInformation(self, port, info):
if port == 0:
info.Set(vtk.vtkDataObject.DATA_TYPE_NAME(),'MyCommandData')
return 1
def RequestData(self, request, inInfo, outInfo):
input_info = inInfo[0].GetInformationObject(0)
input = MyCommandData.GetData(input_info)
output_info = outInfo.GetInformationObject(0)
output = MyCommandData.GetData(output_info)
return 1
a = MyCommand()
b = MyCommand()
b.AddInputConnection(0, a.GetOutputPort())
b.Update()
```
Output is
```auto
2022-01-11 17:34:58.297 ( 0.149s) [4EC3080] vtkDataObjectTypes.cxx:296 WARN| NewDataObject(): You are trying to instantiate DataObjectType "MyCommandData" which does not exist.
2022-01-11 17:34:58.297 ( 0.149s) [4EC3080]vtkDemandDrivenPipeline:615 ERR| vtkCompositeDataPipeline (0x5590416af7b0): Algorithm vtkPythonAlgorithm(0x5590416ae360) did not create output for port 0 when asked by REQUEST_DATA_OBJECT and does not specify a concrete DATA_TYPE_NAME.
```
---
### Author:  [@ben.boeckel](https://discourse.vtk.org/u/ben.boeckel)
#### Post date: [January 21, 2022, 1:24pm UTC](https://discourse.vtk.org/t/vtkdataobject-subclassing-example-python/7619/3 "2022-01-21T13:24:14Z")
It is not possible to subclass VTK objects from Python in ways that the C++ code is aware of them as there is no “glue” class which C+±inherits from arbitrary VTK classes and forwards requests to Python code (as is done for `VTKPythonAlgorithmBase`).