# Python: Reading, Calculate, Write Out
**URL:** https://discourse.vtk.org/t/python-reading-calculate-write-out/1138
**Category:** Support
**Created:** [June 14, 2019, 12:00pm UTC](https://discourse.vtk.org/t/python-reading-calculate-write-out/1138 "2019-06-14T12:00:42Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Horus](https://discourse.vtk.org/u/Horus)
#### Post date: [June 14, 2019, 12:00pm UTC](https://discourse.vtk.org/t/python-reading-calculate-write-out/1138/1 "2019-06-14T12:00:42Z")
Hello,
I want to read in a VTK file, evaluate some function on it and write the file back, including the function result. All that, using the Python VTK interface.
So far I got:
```
from vtk import *
reader = vtkDataSetReader()
reader.SetFileName("bunny-0.1.vtk")
reader.ReadAllVectorsOn()
reader.ReadAllScalarsOn()
reader.Update()
data = reader.GetOutput()
calc = vtkArrayCalculator()
calc.SetInputData(data)
calc.SetFunction("5")
calc.SetResultArrayName("MyResults")
calc.Update()
# Gives: AttributeError: 'NoneType' object has no attribute 'GetPointData'
# print(calc.GetPolyDataOutput().GetPointData().GetArray("MyResults").getValue(10))
writer = vtkUnstructuredGridWriter()
writer.SetInputData(data)
writer.SetFileName("Output.vtk")
writer.Write()
```
It works so far, that it writes out the same geometry to Output.vtk, but without the additional field (here, constant 5).
What is the missing piece?
Thanks!
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [June 14, 2019, 12:35pm UTC](https://discourse.vtk.org/t/python-reading-calculate-write-out/1138/2 "2019-06-14T12:35:05Z")
The problem is that you write the original input to file and not the filter output.
---
### Author:  [@Horus](https://discourse.vtk.org/u/Horus)
#### Post date: [June 14, 2019, 2:08pm UTC](https://discourse.vtk.org/t/python-reading-calculate-write-out/1138/3 "2019-06-14T14:08:24Z")
Ok, thanks!
`writer.SetInputData(calc.GetOutput())`
did the trick.
Another follow up question:
when using the calculator expression: `calc.SetFunction("coordsX+coordsY+coordsZ")`, which works in interactive Paraview I get:
```auto
ERROR: In /build/paraview/src/ParaView-v5.6.0/VTK/Common/Misc/vtkFunctionParser.cxx, line 1244
vtkFunctionParser (0x55eb730b46f0): Syntax error: expecting a variable name; see position 0
```
Thanks!
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [June 14, 2019, 2:20pm UTC](https://discourse.vtk.org/t/python-reading-calculate-write-out/1138/4 "2019-06-14T14:20:47Z")
Check documentation of vtkArrayCalculator. If “follow-up” question is unrelated to the original question (as it was in this case) then please post it as a new topic.
---
### Author:  [@banesullivan](https://discourse.vtk.org/u/banesullivan)
#### Post date: [June 14, 2019, 7:11pm UTC](https://discourse.vtk.org/t/python-reading-calculate-write-out/1138/5 "2019-06-14T19:11:46Z")
Hi @Horus, [PyVista](http://docs.pyvista.org) might be a tool of interest for you. PyVista provides a Pythonic interface to VTK - so you can focus on what’s important; the task at hand, not the nuances of setting up the VTK pipeline. In PyVista, your task would be:
```python
import pyvista as pv
import numpy as np
mesh = pv.read('bunny-0.1.vtk')
mesh['new array'] = np.full(mesh.n_points, 5)
mesh.save('output.vtk')
```
Three tasks, three lines of code 😉 .
If you want to get fancier with summing the coordinates like you tried above, you could try:
```python
mesh['new array'] = np.sum(mesh.points, axis=1)
```
---
### Author:  [@cory.quammen](https://discourse.vtk.org/u/cory.quammen)
#### Post date: [June 14, 2019, 8:08pm UTC](https://discourse.vtk.org/t/python-reading-calculate-write-out/1138/6 "2019-06-14T20:08:55Z")
@Horus
ParaView uses a subclass of `vtkArrayCalculator` to set up some variables to make them available in expressions, including `coordsX`, `coordsY`, and `coordsZ`. To do the same in your VTK program, you can add
```auto
calc.AddCoordinateScalarVariable("coordsX", 0)
calc.AddCoordinateScalarVariable("coordsY", 1)
calc.AddCoordinateScalarVariable("coordsZ", 2)
```
That should do the trick to make your expression work.
As a side note, if you are familiar with NumYy, you may be interested in VTK’s NumPy interface. An excellent tutorial on this interface starts with this blog post by Berk Geveci: [Improved VTK – numpy integration](https://blog.kitware.com/improved-vtk-numpy-integration/).
---
### Author:  [@garcfd](https://discourse.vtk.org/u/garcfd)
#### Post date: [December 12, 2022, 5:57pm UTC](https://discourse.vtk.org/t/python-reading-calculate-write-out/1138/7 "2022-12-12T17:57:25Z")
thanks Bane, thats a really useful example.
as a pyVista “newbie”…
how would I do the exact same, import vtk, calc function, and export vtk
but I want a new function vnew = mag(U) + 10
where mag U is the velocity magnitude, say