This is an implementation of string kernel called "limited range correlation" on Python.

string_kernel.py

from pylab import *

def string_kernel(s1,s2,n=inf):
    k1=[]
    k2=[]
    A=set()
    for k in xrange(1,min(n,len(s1)+1)):
        for i in xrange(0,len(s1)-(k-1)):
            A.add(s1[i:i+k])
    for k in xrange(1,min(n,len(s2)+1)):
        for i in xrange(0,len(s2)-(k-1)):
            A.add(s2[i:i+k])
    for s in A:
        k1.append((1.+log2(len(s)))*s1.count(s))
        k2.append((1.+log2(len(s)))*s2.count(s))
    k1=array(k1)
    k2=array(k2)
    try:
        return k1.dot(k2)/(norm(k1)*norm(k2))
    except ZeroDivisionError:
        return 0.