Your function looks fairly simple to differentiate by hand, but if you have access to the gradient (or you estimate it numerically using scipy...), this function might do the job:

def hessian ( x, the_func, epsilon=1e-8):
"""Numerical approximation to the Hessian
Parameters
------------
x: array-like
The evaluation point
the_func: function
The function. We assume that the function returns the function value and
the associated gradient as the second return element
epsilon: float
The size of the step
"""

N = x.size
h = np.zeros((N,N))
df_0 = the_func ( x )
for i in xrange(N):
xx0 = 1.*x[i]
x[i] = xx0 + epsilon
df_1 = the_func ( x )
h[i,:] = (df_1 - df_0)/epsilon
x[i] = xx0
return h

Jose

On 8 August 2014 08:31, Kiko wrote:
Hi all,

I am trying to calculate a Hessian. I am using numdifftools for this (https://pypi.python.org/pypi/Numdifftools).

My question is, is it possible to make it using pure numpy?.

The actual code is like this:

import numdifftools as nd
import numpy as np

def log_likelihood(params):
sum1 = 0; sum2 = 0
mu = params; sigma = params; xi = params
for z in data:
x = 1 + xi * ((z-mu)/sigma)
sum1 += np.log(x)
sum2 += x**(-1.0/xi)
return -((-len(data) * np.log(sigma)) - (1 + 1/xi)*sum1 - sum2) # negated so we can use 'minimum'

kk = nd.Hessian(log_likelihood)