python 中根据值返回键

>>> dict1 = {"aa":500, "bb":400, "cc":700, "dd":300}
>>> dict1
{'aa': 500, 'cc': 700, 'dd': 300, 'bb': 400}
>>> dict1.keys()[dict1.values().index(300)]       ## 根据值返回键
'dd'
>>> dict1.keys()[dict1.values().index(700)]
'cc'
>>> dict1.keys()[dict1.values().index(400)]
'bb'
>>> dict1.keys()[dict1.values().index(500)]       ## 根据值返回键
'aa'