I'm writing python script to model a part in abaqus

I'm trying to select a datum plan to draw a sketch on it but I can only select the datum plane and datum axis by index numbers , why I can't use findAt command?!!

This is a script example

mdb.models['Model-1'].parts['Sec1'].DatumPlaneByOffset(flip=SIDE2, offset=3.5,     plane=mdb.models['Model-1'].parts['Sec1'].datums[14])