The SetDefault method of the Python dictionary

If the key is in the dictionary, the value corresponding to the key is returned. If the key is not in the dictionary, insert the key into the dictionary and use default as the value of the key and return to default. Default value is None

>>> dict={}
>>> dict[' key ']= ' a '
>>> Dict
{' key ': ' A '}
>>> dict.setdefault (' key ', ' B ') # key exists, so return is simply a.
A
>>> Dict
{' key ': ' A '}
>>> dict.setdefault (' Key0 ', ' B ') # key key0 does not exist, so insert this key and use B as the key value.
' B '
>>> Dict
{' Key0 ': ' B ', ' key ': ' A '}
>>>

The SetDefault method of the Python dictionary