原始代码：

from typing import Dict


related_objects_for_fetch: Dict[str, list] = {}


for key, value in [('k1', 'v1'), ('k1', 'v2'), ('k2', 'v2'), ('k3', 'v3'), ('k2', 'v2')]:
    if key not in related_objects_for_fetch:
        related_objects_for_fetch[key] = []
    if value not in (values := related_objects_for_fetch[key]):
        values.append(value)

for k, v in related_objects_for_fetch.items():
    print(k, v)

修改后：

from collections import defaultdict


related_objects_for_fetch: defaultdict[str, list] = defaultdict(list)


for key, value in [('k1', 'v1'), ('k1', 'v2'), ('k2', 'v2'), ('k3', 'v3'), ('k2', 'v2')]:
    if value not in (values := related_objects_for_fetch[key]):
        values.append(value)


for k, v in related_objects_for_fetch.items():
    print(k, v)

少了if key not in xxx的判断，代码更加简洁。

需要注意的是：

my_dict['not-exist-key']这个会抛出KeyError

而my_defaultdict['not-exist-key']则会返回空列表

如下图：