계층적 데이터가 포함된 중첩 사전에서 Pandas DataFrame을 효율적으로 생성하려면 어떻게 해야 합니까?

user_dict = {
    12: {'Category 1': {'att_1': 1, 'att_2': 'whatever'},
         'Category 2': {'att_1': 23, 'att_2': 'another'}},
    15: {'Category 1': {'att_1': 10, 'att_2': 'foo'},
         'Category 2': {'att_1': 30, 'att_2': 'bar'}}
}

df = pd.DataFrame.from_dict({(i,j): user_dict[i][j] 
                           for i in user_dict.keys() 
                           for j in user_dict[i].keys()},
                       orient='index')

user_ids = []
frames = []

for user_id, d in user_dict.iteritems():
    user_ids.append(user_id)
    frames.append(pd.DataFrame.from_dict(d, orient='index'))

df = pd.concat(frames, keys=user_ids)

               att_1     att_2
12 Category 1      1  whatever
   Category 2     23   another
15 Category 1     10       foo
   Category 2     30       bar