python 效率: counting key occurrences in dict

Question

尝试获取 dict 键的唯一值以及它们在 list 中出现的次数。这有效，但感觉很糟糕:

a = [ {'pid': 1 }, {'pid': 1}, {'pid': 1}, {'pid': 2}, {'pid': 2}, {'pid': 3} ]
b = { x['pid']: len([f for f in a if f['pid'] == x['pid']]) for x in a }
b

产量:

{1: 3, 2: 2, 3: 1}

还有更好的方法吗？

Answer 1

是的，有。使用 collections.Counter :

from collections import Counter

c = Counter(d['pid'] for d in a)
print(c)
# Counter({1: 3, 2: 2, 3: 1})