Python中如何给字典设置默认值

>>> aa = {'a':1, 'b':2}
>>> aa['c']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'c'

>>> from collections import defaultdict
>>> aa = defaultdict(int)
>>> aa['a'] = 1
>>> aa['b'] = 2
>>> aa
defaultdict(<class 'int'>, {'a': 1, 'b': 2})
>>> aa['c']
0
>>> aa
defaultdict(<class 'int'>, {'a': 1, 'b': 2, 'c': 0})
>>> aa = dict.fromkeys('abc', 0)
>>> aa
{'a': 0, 'b': 0, 'c': 0}

>>> aa = defaultdict(lambda : 1)
>>> aa['a']
1

>>> aa = {'a':1, 'b':2}
>>> aa
{'a': 1, 'b': 2}
>>> aa.get('c')
>>> aa.setdefault('c', 'hello')
'hello'
>>> aa.get('c')
'hello'
>>> aa
{'a': 1, 'b': 2, 'c': 'hello'}
>>> aa.setdefault('c', 'world')
'hello'
>>> aa.get('c')
'hello'

>>> aa = {'a':1, 'b':2}
>>> aa
{'a': 1, 'b': 2}
>>> aa['c']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'c'
>>> aa.get('c')
>>> aa
{'a': 1, 'b': 2}
>>> aa.get('c', 'hello')
'hello'
>>> aa.get('b')
2

from collections import defaultdict
data = collections.defaultdict(lambda :[])