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假设一个场景：统计一个列表中每一个数据的数目，要求：尽可能不设定初始值

如果想不出来，可以先看最基础的设定初始值的办法

• 首先需要导入库

  from collections import defaultdict

  原始方案

  user_dict = {}
  users = ["bobby1", "bobby2", "bobby3", "bobby4", "bobby4", "bobby2", "bobby2"]
  for user in users:
  if user not in user_dict:
      user_dict[user] = 1
  else:
      user_dict[user] += 1
  print(user_dict)

改进

使用setdefault函数完成

user_dict = {}
users = ["bobby1", "bobby2", "bobby3", "bobby4", "bobby4", "bobby2", "bobby2"]
for user in users:
user_dict.setdefault(user, 0)
user_dict[user] += 1
print(user_dict)

改进的改进

使用collection中的defaultdict方法

default_dict = defaultdict(int)

users = ["bobby1", "bobby2", "bobby3", "bobby4", "bobby4", "bobby2", "bobby2"]
for user in users:
default_dict[user] += 1
print(default_dict)

# 关于defaultdict方法的补充
> 在初始化并且设定一些特定值的时候可以用到

```python
def gen_default():
    return {
        "name": "",
        "nums": 0
        }

default_dict = defaultdict(gen_default)
print(default_dict["group1"])

• 输出结果

  defaultdict(, {'bobby1': 1, 'bobby2': 3, 'bobby3': 1, 'bobby4': 2})
  {'name': '', 'nums': 0}