# Trouble with vtkPythonAlgorithm
**URL:** https://discourse.paraview.org/t/trouble-with-vtkpythonalgorithm/8992
**Category:** ParaView Support
**Created:** [February 16, 2022, 10:50pm UTC](https://discourse.paraview.org/t/trouble-with-vtkpythonalgorithm/8992 "2022-02-16T22:50:00Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@woodscn](https://discourse.paraview.org/u/woodscn)
#### Post date: [February 16, 2022, 10:50pm UTC](https://discourse.paraview.org/t/trouble-with-vtkpythonalgorithm/8992/1 "2022-02-16T22:50:00Z")
I’m having trouble reproducing the vtkPythonAlgorithm example from here:
[4. Programmable Filter — ParaView Documentation 5.10.0 documentation](https://docs.paraview.org/en/v5.10.0/ReferenceManual/pythonProgrammableFilter.html#python-algorithm)
The plugin appears to load correctly, and I can control the output data type, but the resulting data object is empty. I tested this on a Wavelet source, using 5.10.0 and 5.9.1. Can anyone help me understand what is going on?
```auto
# setup the data processing pipelines
# ----------------------------------------------------------------
# create a new 'Wavelet'
wavelet1 = Wavelet(registrationName='Wavelet1')
# create a new 'Calculator'
calculator1 = Calculator(registrationName='Calculator1', Input=wavelet1)
calculator1.ResultArrayName = 'V'
calculator1.Function = 'RTData'
# create a new 'Half-V Filter'
halfVFilter1 = HalfVFilter(registrationName='HalfVFilter1', Input=calculator1)
```
The plugin source:
```auto
from vtkmodules.vtkCommonDataModel import vtkDataSet
from vtkmodules.util.vtkAlgorithm import VTKPythonAlgorithmBase
from vtkmodules.numpy_interface import dataset_adapter as dsa
# new module for ParaView-specific decorators.
from paraview.util.vtkAlgorithm import smproxy, smproperty, smdomain
@smproxy.filter(label="Half-V Filter")
@smproperty.input(name="Input")
class HalfVFilter(VTKPythonAlgorithmBase):
# the rest of the code here is unchanged.
def __init__ (self):
VTKPythonAlgorithmBase. __init__ (self, outputType='vtkImageData')
def RequestData(self, request, inInfo, outInfo):
# get the first input.
input0 = dsa.WrapDataObject(vtkDataSet.GetData(inInfo[0]))
# compute a value.
data = input0.PointData["V"] / 2.0
# add to output
output = dsa.WrapDataObject(vtkDataSet.GetData(outInfo))
output.PointData.append(data, "V_half");
return 1
```
---
### Author:  [@nicolas.vuaille](https://discourse.paraview.org/u/nicolas.vuaille)
#### Post date: [February 17, 2022, 8:25am UTC](https://discourse.paraview.org/t/trouble-with-vtkpythonalgorithm/8992/2 "2022-02-17T08:25:02Z")
In the plugin code, the output is not initialized… You should add a `ShallowCopy` in RequestData:
```auto
output = dsa.WrapDataObject(vtkDataSet.GetData(outInfo))
output.ShallowCopy(input0.VTKObject)
output.PointData.append(data, "V_half")
```
---
### Author:  [@woodscn](https://discourse.paraview.org/u/woodscn)
#### Post date: [February 17, 2022, 3:42pm UTC](https://discourse.paraview.org/t/trouble-with-vtkpythonalgorithm/8992/3 "2022-02-17T15:42:57Z")
That did it. The documentation on the website should be corrected.
Thanks!
---
### Author:  [@cory.quammen](https://discourse.paraview.org/u/cory.quammen)
#### Post date: [February 17, 2022, 8:52pm UTC](https://discourse.paraview.org/t/trouble-with-vtkpythonalgorithm/8992/4 "2022-02-17T20:52:48Z")
Documentation update happening here: [https://gitlab.kitware.com/paraview/paraview-docs/-/merge\_requests/30](https://gitlab.kitware.com/paraview/paraview-docs/-/merge_requests/30)