Python 字典操作

company={}
news_date="2018-02-23"
title="asdasdad"
company[news_date]=title
company
{'2018-02-23': 'asdasdad'}
company.setdefault(news_date,{}).setdefault("asdasasdasdasdqe121")
Traceback (most recent call last):
File "<input>", line 1, in <module>
AttributeError: 'str' object has no attribute 'setdefault'
company.setdefault(news_date,{"asdasd"})
'asdasdad'
company
{'2018-02-23': 'asdasdad'}
company.setdefault(news_date,{"asdasdasdada"})
'asdasdad'

如何改变 news_date 的 key 为字典类型