import vtk

r = vtk.vtkImageReader()
r.SetFileDimensionality(3)
# I'm hoping that you don't really have this file lying around
r.SetFileName('meeeeeep.raw')

# define the observer
def observerR(eventObject, eventName, CallData):
    print "CallData is %s" % (CallData,)
    
# set the CallDataType attribute
observerR.CallDataType = 'string0'
    
r.AddObserver('ErrorEvent', observerR)

# this should trigger the error
r.Update()