Quote:A set object is an unordered collection of distinct hashable objects.
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There are currently two built-in set types, set and frozenset. The set type is mutable — the contents can be changed using methods like add() and remove(). Since it is mutable, it has no hash value and cannot be used as either a dictionary key or as an element of another set. The frozenset type is immutable and hashable — its contents cannot be altered after it is created; it can therefore be used as a dictionary key or as an element of another set.

>>> spam = {1, 2}
>>> type(spam)
<class 'set'>
>>> eggs = {spam}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'set'
>>> spam = frozenset(spam)
>>> type(spam)
<class 'frozenset'>
>>> eggs = {spam}
>>> type(eggs)
<class 'set'>

a = {1, 2, 3}
b = frozenset(a)
print(a.__hash__, b.__hash__)

Output:
None <method-wrapper '__hash__' of frozenset object at 0x0000027837D3F120>