"""
setdefault方法参数输入已有键，返回对应值，找不到已有键，创建新键，值为None
"""
>>> dict4
{‘abc‘: ‘abc‘}
>>> dict4.setdefault(‘aaa‘)
>>> dict4
{‘abc‘: ‘abc‘, ‘aaa‘: None}
>>> dict4.setdefault(‘abc‘)
‘abc‘
>>> dict4
{‘abc‘: ‘abc‘, ‘aaa‘: None}
"""
setdefault方法参数输入已有键，返回对应值，找不到已有键，创建新键，值为None
"""
>>> dict4
{‘abc‘: ‘abc‘}
>>> dict4.setdefault(‘aaa‘)
>>> dict4
{‘abc‘: ‘abc‘, ‘aaa‘: None}
>>> dict4.setdefault(‘abc‘)
‘abc‘
>>> dict4
{‘abc‘: ‘abc‘, ‘aaa‘: None}
原文：https://www.cnblogs.com/wawawawa-briefnote/p/8588046.html