This calculates the Levenshtein distance between a and b.

def levenshtein_distance(self, a, b):
        '''This calculates the Levenshtein distance between a and b.
        '''

        n, m = len(a), len(b)
        if n > m:
            a,b = b,a
            n,m = m,n
        current = range(n+1)
        for i in range(1,m+1):
            previous, current = current, [i]+[0]*n
            for j in range(1,n+1):
                add, delete = previous[j]+1, current[j-1]+1
                change = previous[j-1]
                if a[j-1] != b[i-1]:
                    change = change + 1
                current[j] = min(add, delete, change)
        return current[n]