The setdefault() method in Python allows you to get the value of a key if it exists in the dictionary, and if not, it inserts the key with a default value and returns that value. It is very useful when you don't want to manually check if a key exists before adding or updating it.
In the following example, I declared a dictionary 'aa'. Using the setdefault() method, I am either adding a value to an existing key or adding a new key with some value.
>> aa={}
>>> aa.setdefault('a',set()).add(2)
>>> aa
{'a': {2}}
>>> aa.setdefault('a',set()).add(3)
>>> aa
{'a': {2, 3}}
>>> aa.setdefault('b',set()).add(13)
>>> aa
{'a': {2, 3}, 'b': {13}}