>>> list1=[3,2,1,1,2,3,4,5]
>>> set(list1)
{1, 2, 3, 4, 5}
>>> list2=[3,4,5,6,7,8]
>>> set(list1).add(2)
>>> set(list1).add(6)
>>> print(set(list1).add(2))
None
>>> print(set(list1).add(6))
None
>>> set1=set(list1)
>>> set2=set(list2)
>>> set1,set2
({1, 2, 3, 4, 5}, {3, 4, 5, 6, 7, 8})
>>> set1.add(3)
>>> print(set1.add(3)
... )
None
>>> print(set1.add(7))
None
>>> set1.add('aaa')
>>> set1
{1, 2, 3, 4, 5, 7, 'aaa'}
>>> id(set1)
140138768484616
>>> set1.add('aaaa')
>>> id(set1)
140138768484616
>>> set1
{1, 2, 3, 4, 5, 7, 'aaa', 'aaaa'}
>>> set1.add('7')
>>> set1
{1, 2, 3, 4, 5, 7, 'aaa', 'aaaa', '7'}
>>> set1.add(7)
>>> set1
{1, 2, 3, 4, 5, 7, 'aaa', 'aaaa', '7'}