python对字典降序排序

def test2():    a = [['一','一','二'],['一','二','三']]    for docno, document in enumerate(a):        print docno,','.join(document)    counter = defaultdict(int)    for w in sum(a,[]):        counter[w if isinstance(w, unicode) else unicode(w, 'utf-8')] += 1    print counter    print iteritems(counter)    print sum(itervalues(counter))    result = sorted(iteritems(counter), key=lambda d:d[1], reverse = True)    print result