# Using VTK gradient filter to calculate the divergence of a vector field
**URL:** https://discourse.vtk.org/t/using-vtk-gradient-filter-to-calculate-the-divergence-of-a-vector-field/5319
**Category:** Support
**Created:** [March 4, 2021, 5:17pm UTC](https://discourse.vtk.org/t/using-vtk-gradient-filter-to-calculate-the-divergence-of-a-vector-field/5319 "2021-03-04T17:17:12Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Renard](https://discourse.vtk.org/u/Renard)
#### Post date: [March 4, 2021, 5:17pm UTC](https://discourse.vtk.org/t/using-vtk-gradient-filter-to-calculate-the-divergence-of-a-vector-field/5319/1 "2021-03-04T17:17:12Z")
Hi, I have written a simple code in python to calculate the divergence of a vector field stored in a VTU file. I’m getting the following error, what is going wrong here?
TypeError: method requires a string argument
```auto
import numpy
import vtk
def GetField(vtu_filename, fieldname):
""" This will extract the field with the given name from the specified file """
reader = vtk.vtkXMLUnstructuredGridReader()
reader.SetFileName(vtu_filename)
reader.Update()
field_array = reader.GetOutput().GetPointData().GetArray(fieldname)
return field_array
def ComputeDivergence(input_array):
gradient_9d = vtk.vtkGradientFilter(input_array)
du_dx, dv_dy, dw_dz = gradient_9d[0], gradient_9d[4], gradient_9d[8]
return du_dx + dv_dy + dw_dz
Myfile = "hys_Ldiamond080.vtu"
Magnetization = GetField(Myfile, "Magnetization")
divergence = ComputeDivergence(Magnetization)
```
Full Error :
```auto
Traceback (most recent call last):
File "Stack.py", line 20, in
divergence = ComputeDivergence(Magnetization)
File "Stack.py", line 12, in ComputeDivergence
gradient_9d = vtk.vtkGradientFilter(input_array)
TypeError: method requires a string argument
```
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [March 4, 2021, 11:09pm UTC](https://discourse.vtk.org/t/using-vtk-gradient-filter-to-calculate-the-divergence-of-a-vector-field/5319/2 "2021-03-04T23:09:38Z")
We cannot guess where the problem occurs from this trimmed down error message. Please provide complete error message - including call stack and line numbers.
---
### Author:  [@Renard](https://discourse.vtk.org/u/Renard)
#### Post date: [March 5, 2021, 9:19am UTC](https://discourse.vtk.org/t/using-vtk-gradient-filter-to-calculate-the-divergence-of-a-vector-field/5319/3 "2021-03-05T09:19:08Z")
Hi, I have edited the post.