users = ['bobby1', 'bobby2', 'bobby3', 'bobby1', 'bobby2', 'bobby2' ]統計各個元素出現次數

1.傳統方法

user_dict = {}
for user in users:
    if user not in user_dict:
        user_dict[user] = 1
    else:
        user_dict[user] +=1

2.用setdefault方法，當元素沒有在字典中出現時設定一個預設值

for user in users:
    user_dict.setdefault(user, 0) # 程式碼簡潔、且效能高
    user_dict[user] +=1
print user_dict
 

3.用defaultdict，defaultdict接收一個可呼叫變數來做初始化

from collections import defaultdict
# 進一步優化，程式碼量更少
default_dict = defaultdict(int) # 傳一個可呼叫的物件 list int
for user in users:
    default_dict[user] +=1

提高：已知s，輸出結果為[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

s = [('yellow',1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d = defaultdict(list)
for color, c in s:
    d[color].append(c)
print d.items()