Clipping by Scalar from CellData / vtkClipDataSet / Python

I want to read a .vtu file and clip it so that I can work with the necessary data only. To do so the .vtu contains CellData with a tag which I can use to clip by scalars. After some searching I came to the following code but it seems that clip.GetOutput( ) is empty. Can you help me?
I tried a lot but since I am new to VTK I can’t find a solution.

#file path
filename = ‘test.vtu’

#read vtk returns vtkreader (a function I wrote in an other module)
file = f.readData( filename )

#clip only necessary data
tags = file.GetOutput( ).GetCellData( ).GetArray( ‘Tag’ )
file.GetOutput( ).GetCellData( ).SetScalars( tags )

clip = vtk.vtkClipDataSet( )
clip.SetInputData( file.GetOutput( ) )
clip.SetValue( 100 )
clip.SetInsideOut( True )
clip.Update( )

Could you elaborate on what result you are expecting here? By clipping do you mean extracting from the mesh a submesh that would contain all the cells of a given cell value?

Yes, I want a submesh separated from the rest by a certain tag value that is stored in the cells.
But I already found a solution. I am using vtk.Treshold( ) with an upper bound for the tag values. This works fine for my case.