A data extractor that would choose the correct writer type based on input data type would be a welcomed addition. This could take the form of an extractor of type Data/auto. Such a feature would make in-situ post-processing with ParaView-Catalyst smoother.
Let me give a couple examples:
I generally setup my in-situ post-processing pipeline by loading a previous result file into ParaView. With the disk reader I am using, the data in loaded into ParaView as a MultiBlockDataSet. If I setup a slice and an extractor for that slice, ParaView invites me to use an extractor of type VTM (in fact, other data extractors are grayed-out as ParaView detects my data type). However, Catalyst 2.0 will send data of type vtkPartionedDataSet or vtkPartionedDataSetCollection. I therefore have to change the writer in the Catalyst script to a VTPC.
Say I now want to use a merge block on my slice. Knowing the syntax, I can add it directly in the python script, without opening ParaView again. I now also need to remember to change the writer for the slice to a VTU writer.
Having a data writer of type AUTO would make this smoother, especially for users not familiar with the different data types in ParaView.