This may be old news for most people using Numeric Python on a daily basis, but I've just "discovered" that Numeric ufuncs can be applied to class instances, if they implement the right interface, which is a method with the same name as the ufunc.

One dumb example :

class ZeroAngle:
    def sin(self):
        return 0.
    def tan(self):
        return 0
    def cos(self):
        return 1

 a = ZeroAngle()
import Numeric as n
print n.cos(za)
print n.sin(za)

This of course does not work with the functions in math or cmath, which try to coerce there argument to float.

A very nice usage of this is in Konrad Hinsen's Scientific.Functions.Derivatives (part of the Scientific Python package)