a = set().add(1)
print a # Prints `None`

>>> a_list = [3, 2, 1]
>>> print a_list.sort()
None
>>> a_list
[1, 2, 3]

>>> a_dict = {}
>>> print a_dict.__setitem__('a', 1)
None
>>> a_dict
{'a': 1}

>>> a_set = set()
>>> print a_set.add(1)
None
>>> a_set
set([1])

a = set()
a.add(1)

a = set([1])

>>> empty = set()
>>> print(empty.add(1))
None
>>> empty
set([1])

a = set()
a.add(1)
print a

a = set([1, 2, 3])

a = set()
a.add(1)
print a

a = set()
a.add(1)
print(a)

a = {1}
print(a)

a = set()
a = set() | {1}