Hi Tancredi,
see my reply at Running fast-jet and fast-jet contrib in RDataFrame - #3 by eguiraud regarding Python functions and RDF.
We expect to make this more nice and flexible in the future, but basically the requirement for injecting pure Python code in the RDF event loop is that it must be compilable to machine code by numba (with nopython=True).
Cheers,
Enrico