If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.
如果键在字典中，返回这个键所对应的值。如果键不在字典中，向字典 中插入这个键，并且以default为这个键的值，并返回 default。default的默认值为None
Python 2.5.4 (r254:67916, Dec 23 2008, 15:19:34) [MSC v.1400 64 bit (AMD64)] on win32
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IDLE 1.2.4
>>> dict={}
>>> dict['key']='a'
>>> dict
{'key': 'a'}
>>> dict.setdefault('key', 'b') # 键key存在，故返回简直a.
'a'
>>> dict
{'key': 'a'}
>>> dict.setdefault('key0', 'b') # 键key0不存在，故插入此键，并以b为键值.
'b'
>>> dict
{'key0': 'b', 'key': 'a'}
>>>