Function Attributes assigned by decorator

It appears to be straight forward to initialize function attributes by a decorator (the setup function).
The new function factory looks a lot like the class based version of my previous post, and performs similarly.

def make_standard_deviator2():
    '''Generate functions that return running standard deviations
    Uses function attributes applied by decorator
    '''

    def setup(func):
      func.N, func.X, func.X2 = [0.0]*3
      return func

    @setup
    def sd(x):
        '''returns standard deviation.
        Uses function attributes holding running sums
        '''
        from math import sqrt

        sd.N += 1       # Num values
        sd.X += x       # sum values
        sd.X2 += x**2   # sum squares

        if sd.N<2: return 0.0
        return sqrt((sd.N*sd.X2 - sd.X**2)
                    /(sd.N*(sd.N-1)))

    return sd