python 字典defaultdict(list)

假如有需求，

data = [("p", 1), ("p", 2), ("p", 3),
        ("h", 1), ("h", 2), ("h", 3)]

要转换成

result = {'p': [1, 2, 3], 'h': [1, 2, 3]}

我的第一想法就是这么做

result = {}
for (key, value) in data:
    if key in result:
        result[key].append(value)
    else:
        result[key] = [value]

师傅看了之后笑了，扔来一段

result = {}
data = [("p", 1), ("p", 2), ("p", 3),
        ("h", 1), ("h", 2), ("h", 3)]
for (key, value) in data:
    result.setdefault(key, []).append(value)
print(result)#'p': [1, 2, 3], 'h': [1, 2, 3]}

 setderault()方法接收两个参数，用法和get类似，但比get强大，它可以给字典的key设定一个默认值（如果key不在字典中的时候），

defaultdict(list),会构建一个默认value为list的字典，

from collections import defaultdict
result = defaultdict(list)
data = [("p", 1), ("p", 2), ("p", 3),
        ("h", 1), ("h", 2), ("h", 3)]

for (key, value) in data:
    result[key].append(value)
print(result)#defaultdict(<class 'list'>, {'p': [1, 2, 3], 'h': [1, 2, 3]})