vtkDataSet.SafeDownCast probleam

I wrote a statement ds = vtk.vtkDataSet.SafeDownCast(reader.GetOutput().GetBlock(i)) in python. I want to convert this data to vtkDataSet type but the converted ds is NoneType. I don’t know what went wrong, can anyone tell me something.By the way reader.GetOutput().GetBlock(i) output type is vtkMultiBlockDataSet

If the block is a vtkMultiBlockDataSet then you will also have to iterate through its blocks (possibly recursively) until you finally find a vtkDataSet.

Note that vtkMultiBlockDataSet is not a subclass of vtkDataSet.

I do not know how can i do will solve it,so can you tell me what should i do.Thank you