正常字典在查询不存在的键后引发 Keyerror

>>> from collections import defaultdict
>>> my_dict = {"one": 1, "two": 2}
>>> my_dict["three"]
Traceback (most recent call last):
  File "", line 1, in 
KeyError: 'three'

但是通过在 deafultdict 中使用 lamda，我们可以为未定义的键设置默认值

# create a default dict, from a dict
>>> my_def_dict = defaultdict(lambda: -1, my_dict)
>>> my_def_dict["zero"]
-1

# create an empty default dict
>>> empty_def_dict = defaultdict(lambda: true)
# add key-value pairs here