Oh, interesting - I didn't realize there existed a Parquet source in scalding already, and I wrote my own to test this (I'm working on a project using scalding and this). 

Seems reasonable to add an empty constructor and then throw an exception if you try to use it as a sink without specifying the class. Then we can also pass it down to ThriftReadSupport (if it's specified) - I should've added that anyway. Does that work for you guys?