Are Python sets mutable?

Question

>>>> x = set([1, 2, 3])
>>>> y = x
>>>> 
>>>> y |= set([4, 5, 6])

>>>> print x
set([1, 2, 3, 4, 5, 6])
>>>> print y
set([1, 2, 3, 4, 5, 6])

Sets are unordered.
Set elements are unique. Duplicate elements are not allowed.
A set itself may be modified, but the elements contained in the set must be of an immutable type.

set1 = {1,2,3}

set2 = {1,2,[1,2]}  --> unhashable type: 'list'
# Set elements should be immutable.

Conclusion: sets are mutable.