Error while passing a VTK object in VmtkNetworkExtraction

Hello Everyone,
I’m trying to skeletonize a 3D volume n VMTK using the following code in python

vmtkCommand = '''vmtksurfacereader -ifile foo.stl
                 --pipe vmtksurfacewriter -ofile foo_net.vtp'''

p = pypes.PypeRun(vmtkCommand)

networkExtraction = vmtkscripts.vmtkNetworkExtraction()
networkExtraction.Surface = 'foo_net.vtp'
network = networkExtraction.Network

But I get the following error while passing surface in the line networkExtraction.Surface = ‘foo_net.vtp’.

line 23, in <module>
TypeError: SetInputData argument 1: method requires a VTK object

Any suggestions to how to resolve this error will be really helpful.

'foo_net.vtp' is a string. You’ll need to use a reader to load the file then connect that reader with the extraction object in a pipeline.