可以定义一个字典，可以添加默认值，int 为 0，list 为 []，set 为 {}

int：默认值为 0

from collections import defaultdict
int_dict = defaultdict(int)

for i in range(5):
    int_dict[i] += i
    
int_dict

output:
defaultdict(int, {0: 0, 1: 1, 2: 2, 3: 3, 4: 4})

list，默认值为空列表，也可以用 append

from collections import defaultdict
list_dict = defaultdict(list)

for i in range(5):
    list_dict[i] += [i]
    
list_dict

output:
defaultdict(list, {0: [0], 1: [1], 2: [2], 3: [3], 4: [4]})

set：默认值为空集合

from collections import defaultdict
int_dict = defaultdict(set)

for i in range(5):
    int_dict[i].add(i)
    
int_dict

output:
defaultdict(set, {0: {0}, 1: {1}, 2: {2}, 3: {3}, 4: {4}})