writer = reader[0]

writer = CreateWriter('Difference,e', reader)

writer.CellData.append( reader[0].CellData['x'] - reader[1].CellData['x'] , 'x')

Traceback (most recent call last):
   File "<console>", line 1, in <module>
  File "C:Program FilesParaView 5.2.0-Qt4-OpenGL2-Windows-64bitbinlibsite-packagesparaviewservermanager.py", line 1597, in __getattr__
    raise AttributeError("class has no attribute %s" % name)
AttributeError: class has no attribute append