[Paraview] small ParaView Script not working

[Marco Kokic]

Dear ParaViewers,

I have written and tested the following script, which I hope is 
self-explanatory. It doesn't work though (print cells_ gives "None") and 
I can't see what I missed. I'm thankful for any suggestions.

#!/usr/bin/pvpython

#setup paraview connection
from paraview.simple import *

def my_range(start, end, step):
     while start <= end:
         yield start
         start += step

def LoadMultipleFiles(FilePrefix, Low, High):
     for i in my_range(Low,High+1,100):
         reader = XMLMultiBlockDataReader(FileName='/path-to-files/' + 
FilePrefix + str(i) + '.vtm')

cells_ = LoadMultipleFiles('Cells_', 0, 1000)
print cells_

# create a new 'Integrate Variables'
#integrateVariables1 = IntegrateVariables(Input=cells_)
# create a new 'Plot Selection Over Time'
#plotSelectionOverTime1 = 
PlotSelectionOverTime(Input=integrateVariables1,Selection=None)
# save data
#SaveData('/path-to-files/output10_areaovertime.csv', 
proxy=plotSelectionOverTime1, 
Precision=5,UseScientificNotation=0,WriteAllTimeSteps=0)