# Help with Python Plugin Filter using VTKPythonAlgorithmBase approach

**URL:** https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503
**Category:** Development
**Created:** [September 3, 2019, 1:01pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503 "2019-09-03T13:01:29Z")
**Posts on this page:** 9
**Page:** 1

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### Author: ![vchan1186](https://discourse.paraview.org/letter_avatar_proxy/v4/letter/v/ed8c4c/32.png) [@vchan1186](https://discourse.paraview.org/u/vchan1186)
#### Post date: [September 3, 2019, 1:01pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/1 "2019-09-03T13:01:29Z")

</div>

I am trying to use the VTKPythonAlgorithmBase approach to create a custom filter plugin that will send the filter output back to the ParaView pipeline.

I’ve adopted the “Half V Filter” example in the chapter 12.4 of the ParaView guide for my particular dataset:

```python
from vtkmodules.vtkCommonDataModel import vtkDataSet
from vtkmodules.util.vtkAlgorithm import VTKPythonAlgorithmBase
from vtkmodules.numpy_interface import dataset_adapter as dsa
from paraview.util.vtkAlgorithm import smproxy, smproperty, smdomain

@smproxy.filter(label="Half ID Filter")
@smproperty.input(name="Input")
class HalfIDFilter(VTKPythonAlgorithmBase):
    def __init__ (self):
        VTKPythonAlgorithmBase. __init__ (self)
    def RequestData(self, request, inInfo, outInfo):
        input0 = dsa.WrapDataObject(vtkDataSet.GetData(inInfo[0]))
        data = input0.CellData["ID"]/2.0
        output = dsa.WrapDataObject(vtkDataSet.GetData(outInfo))
        output.CellData.append(data, "ID_half")
        return 1

```

The above filter is applied to a data set that has been loaded into the pipeline. The data set contains cell data named “ID”. I’m hoping to apply this filter and get a new cell data entry named “ID\_half”.

Currently, I am able to load the filter and apply it. However, nothing is outputted into the pipeline.

Thank you,  
Victor

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<div class="post-metadata">

### Author: ![aron.helser](https://discourse.paraview.org/user_avatar/discourse.paraview.org/aron.helser/32/13_2.png) [@aron.helser](https://discourse.paraview.org/u/aron.helser)
#### Post date: [September 3, 2019, 2:47pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/2 "2019-09-03T14:47:11Z")

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That certainly looks reasonable to me. It’s not obvious, but if you add ‘print’ statements to your filters, you can see them in the Output window.  
In my filter, I was unable to use vtkDataSet.GetData(outInfo), I had to use a more specific type like vtkPolyData, but the example clearly uses vtkDataSet, and I’m not using numpy.

I suggest adding `print(output)` to see if it’s doing what you want.

HTH,

Aron

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<div class="post-metadata">

### Author: ![vchan1186](https://discourse.paraview.org/letter_avatar_proxy/v4/letter/v/ed8c4c/32.png) [@vchan1186](https://discourse.paraview.org/u/vchan1186)
#### Post date: [September 3, 2019, 4:41pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/3 "2019-09-03T16:41:07Z")

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Hi Aron,  
When printing the output in the custom filter above, I see

```python
<vtkmodules.numpy_interface.dataset_adapter.PolyData object at ...>

```

However, when I do the same in the following programmable filter (which accomplishes what I’m hoping for in the above custom filter):

```python
input0 = inputs[0]
data = input0.CellData["ID"]/2.0
output.CellData.append(data, "ID_half")
print(output)

```

I get

```python
<vtkmodules.numpy_interface.dataset_adapter.UnstructuredGrid object at ...>

```

I notice that in the custom filter, `output` is a PolyData object, while in the programmable filter, `output` is an UnstructuredGrid object. I wonder if this is the reason the output is not sent back to the ParaView data pipeline.

Is there a way to make the `outInfo` variable in `RequestData(...)` to be the same type of object as the input, i.e., an UnstructuredGrid object?

Thanks,  
Victor

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<div class="post-metadata">

### Author: ![aron.helser](https://discourse.paraview.org/user_avatar/discourse.paraview.org/aron.helser/32/13_2.png) [@aron.helser](https://discourse.paraview.org/u/aron.helser)
#### Post date: [September 3, 2019, 5:11pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/4 "2019-09-03T17:11:43Z")

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Hi Victor,  
If you go back to the ParaView Guide 12.4, you’ll see some of the examples specify the output type:

VTKPythonAlgorithmBase. **init** (self, nInputPorts=1, nOutputPorts=1, outputType=“vtkUnstructuredGrid” )

Hopefully that will fix your problem.

Regards,

Aron

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<div class="post-metadata">

### Author: ![vchan1186](https://discourse.paraview.org/letter_avatar_proxy/v4/letter/v/ed8c4c/32.png) [@vchan1186](https://discourse.paraview.org/u/vchan1186)
#### Post date: [September 3, 2019, 6:12pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/5 "2019-09-03T18:12:45Z")

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Hi Aron,  
Making the object type consistent did not solve the problem. I still cannot access the output data in the ParaView data pipeline. I have rewritten the custom filter as follows:

```python
from vtkmodules.vtkCommonDataModel import vtkDataSet, vtkUnstructuredGrid
from vtkmodules.util.vtkAlgorithm import VTKPythonAlgorithmBase
from vtkmodules.numpy_interface import dataset_adapter as dsa
from paraview.util.vtkAlgorithm import smproxy, smproperty, smdomain

@smproxy.filter(label="Half ID Filter")
@smproperty.input(name="Input")
class HalfIDFilter(VTKPythonAlgorithmBase):
    def __init__ (self):
        VTKPythonAlgorithmBase. __init__ (self, 
                                        nInputPorts=1, 
                                        nOutputPorts=1, 
                                        outputType="vtkUnstructuredGrid")
    def RequestData(self, request, inInfo, outInfo):
        input0 = dsa.WrapDataObject(vtkUnstructuredGrid.GetData(inInfo[0]))
        data = input0.CellData["ID"]/2.0
        output = dsa.WrapDataObject(vtkUnstructuredGrid.GetData(outInfo))
        output.CellData.append(data, "ID_half")
        print("output object = ", output)
        print("output keys = ", output.CellData.keys())
        print("output data = ", output.CellData["ID_half"])
        return 1

```

Note that all the `vtkDataSet` calls have now been switched to `vtkUnstructuredGrid`. The output in the output window when applying the filter is

```python
('output object = ', <vtkmodules.numpy_interface.dataset_adapter.UnstructuredGrid object at ... >)
('output keys = ', ['ID_half'])
('output data = ', VTKArray([], shape=(0L, 534L), dtype=float64)

```

As seen from the output, the object type is `UnstructuredGrid`, the `"ID_half"` key has been added, but the data in the output object is empty.

Thanks,  
Victor

---

<div class="post-metadata">

### Author: ![aron.helser](https://discourse.paraview.org/user_avatar/discourse.paraview.org/aron.helser/32/13_2.png) [@aron.helser](https://discourse.paraview.org/u/aron.helser)
#### Post date: [September 3, 2019, 6:25pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/6 "2019-09-03T18:25:23Z")

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I’m pretty sure you can fix this problem by adding this just after output is defined:  
output.ShallowCopy(input0)

I’m not sure why the example from the ParaView guide doesn’t include this, too.

Regards,

Aron

---

<div class="post-metadata">

### Author: ![vchan1186](https://discourse.paraview.org/letter_avatar_proxy/v4/letter/v/ed8c4c/32.png) [@vchan1186](https://discourse.paraview.org/u/vchan1186)
#### Post date: [September 3, 2019, 6:58pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/7 "2019-09-03T18:58:59Z")

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Hi Aron,  
The shallow copy did the job of exposing the output of the filter to the ParaView data pipeline. I ended up performing the shallow copy before the append call as follows:

```python
output = dsa.WrapDataObject(vtkUnstructuredGrid.GetData(outInfo))
output.ShallowCopy(vtkUnstructuredGrid.GetData(inInfo[0]))
output.CellData.append(data, "ID_half")

```

However, wouldn’t the shallow copy end up doubling my memory footprint? Is there a way to only expose the newly created data set.

Thanks,  
Victor

---

<div class="post-metadata">

### Author: ![aron.helser](https://discourse.paraview.org/user_avatar/discourse.paraview.org/aron.helser/32/13_2.png) [@aron.helser](https://discourse.paraview.org/u/aron.helser)
#### Post date: [September 3, 2019, 7:32pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/8 "2019-09-03T19:32:39Z")

</div>

No, it won’t increase your memory footprint - that is the purpose of ShallowCopy (instead of DeepCopy). The output will use references to all the input data structures, except for the one ID\_half array you have added to the output data.

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<div class="post-metadata">

### Author: ![vchan1186](https://discourse.paraview.org/letter_avatar_proxy/v4/letter/v/ed8c4c/32.png) [@vchan1186](https://discourse.paraview.org/u/vchan1186)
#### Post date: [September 3, 2019, 7:34pm UTC](https://discourse.paraview.org/t/help-with-python-plugin-filter-using-vtkpythonalgorithmbase-approach/2503/9 "2019-09-03T19:34:11Z")

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Got it. Thank you for all your help Aron!