Python字典增量

Question

在Python中,在增加密钥之前必须首先检查密钥是否在字典中是很烦人的:

if key in my_dict:
  my_dict[key] += num
else:
  my_dict[key] = num

以上四条线是否有较短的替代品？

Answer 1

另一种选择是:

my_dict[key] = my_dict.get(key, 0) + num

Answer 2

你有很多选择.我喜欢用Counter:

>>> from collections import Counter
>>> d = Counter()
>>> d[12] += 3
>>> d
Counter({12: 3})

或者defaultdict:

>>> from collections import defaultdict
>>> d = defaultdict(int)  # int() == 0, so the default value for each key is 0
>>> d[12] += 3
>>> d
defaultdict(<function <lambda> at 0x7ff2fe7d37d0>, {12: 3})

Answer 3

你想要什么叫做defaultdict

请参阅http://docs.python.org/library/collections.html#collections.defaultdict

Answer 4

转变:

if key in my_dict:
  my_dict[key] += num
else:
  my_dict[key] = num

进入以下用途setdefault:

my_dict[key] = my_dict.setdefault(key, 0) + num