Simple unweighted Levenshtein distance

def _editdistance(a, b):
        """ Simple unweighted Levenshtein distance """
        r0 = range(0, len(b) + 1)
        r1 = [0] * (len(b) + 1)

        for i in range(0, len(a)):
            r1[0] = i + 1

            for j in range(0, len(b)):
                c = 0 if a[i] is b[j] else 1
                r1[j + 1] = min(r1[j] + 1, r0[j + 1] + 1, r0[j] + c)

            r0 = r1[:]

        return r1[len(b)]