Python 檢查 dict 的索引值是否存在

Check if a given key exists in a dict

要檢查 dictionary 是否存在索引值
直接查會直接 KeyError

>>> d = {}
>>> d['a']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'a'

這時就要靠 collections.defaultdict(default_factory)

>>> from collections import defaultdict
>>> d = defaultdict(int)
>>> d['a']
0

再回到檢查 dictionary 是否存在索引值的地方

from collections import defaultdict

s = 'mississippi'
my_dict = defaultdict(int) 
for key in s:
     if key in my_dict:
          my_dict[key] += 1
     else:
          my_dict[key] = 1

print(my_dict)

結果

defaultdict(, {'m': 1, 'i': 4, 's': 4, 'p': 2})

參考資料 :
https://docs.python.org/3/library/collections.html#defaultdict-examples
https://stackoverflow.com/questions/473099/check-if-a-given-key-already-exists-in-a-dictionary-and-increment-it