Proximity in hierarchy

In function cluster_dotted_names, local proximity function code is :

def proximity(col_a, col_b):
    return(sum(a==b for a,b in zip(col_a, col_b)))
# proximity ('a.b.c.d.e','b.b.c.d.e') => 8

Since a dotted name reflects position in a hierarchy, should the count stop at the first difference when reading from left to right ?

def proximity(col_a, col_b):
    res = 0;
    for (a,b) in zip(col_a, col_b):
        if (a==b):
            res = res + 1
        else:
            break;
    return res
# proximity('a.b.c.d.e','b.b.c.d.e') => 0