统计list元素个数

两种方法：

1. from collections import Counter
2. 一一列举

举例：

l = [1,3,4,7,3,2,6,9,5,0,3,6,1,6,3,8,6,7,2,5]

#item_id_list: 12万
def counter(item_id_list):
   return Counter(item_id_list)

def count(item_id_list):
   dict_item = {}
   for item in item_id_list:
       dict_item[item] = dict_item.get(item, 0) + 1
   return dict_item
s = item_id_list[:20]
t1= timeit.Timer('counter(s)', 'from __main__ import counter, s')
t2 = timeit.Timer('count(s)', 'from __main__ import count, s')
print(t1.timeit(number=1000))
print(t2.timeit(number=1000)) 
           

len(list)	20	200	2000	20000
Counter	0.01318	0.02240	0.11282	1.0403
一一列举	0.01370	0.03722	0.29194	2.7899

结论：元素个数越多，collections.Counter统计元素个数所需时间越少