I agree that there is a place for material that is not entirely for the beginner and I too wondered  whether putting this in tutorial is best.  I feel that the finite difference of derivatives is such a basic need of practicing computational scientists (especially those who need to devise methods for simulating the solutions of pde and ode) that it seems clear to me that SymPy would be performing a good service with this kind of material somewhere.  I am open to put it wherever the community thinks it would best serve. I just hope people agree that it is worth doing.   Thanks for comments and suggestions.