standard deviation of the values

std2.py

import numpy as np

def std2(x):
    sum, sum2, n = 0.0, 0.0, 0.0
    for k in x:
        sum += k
        sum2 += k*k
        n += 1
    return sum2/n - sum*sum/(n*n)

a = [1,2,3,4,5,6,7,8,9]

## Ref http://docs.scipy.org/doc/numpy/reference/generated/numpy.std.html
print np.std(a)**2 - std2(a)