Are Python sets mutable?

Question

Are sets in Python mutable?

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In other words, if I do this:

x = set([1, 2, 3])
y = x

y |= set([4, 5, 6])

Are x and

Answer 1

Sets are muttable

s = {2,3,4,5,6}
type(s)
<class 'set'>
s.add(9)
s
{2, 3, 4, 5, 6, 9}

We are able to change elements of set

Answer 2

>>>> 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.

Answer 3

After changing the set, even their object references match. I don't know why that textbook says sets are immutable.

    >>> s1 ={1,2,3}
    >>> id(s1)
    140061513171016
    >>> s1|={5,6,7}
    >>> s1
    {1, 2, 3, 5, 6, 7}
    >>> id(s1)
    140061513171016

Answer 4

I don't think Python sets are mutable as mentioned clearly in book "Learning Python 5th Edition by Mark Lutz - Oreilly Publications"

Answer 5

Python sets are classified into two types. Mutable and immutable. A set created with 'set' is mutable while the one created with 'frozenset' is immutable.

>>> s = set(list('hello'))
>>> type(s)
<class 'set'>

The following methods are for mutable sets.

s.add(item) -- Adds item to s. Has no effect if listis already in s.

s.clear() -- Removes all items from s.

s.difference_update(t) -- Removes all the items from s that are also in t.

s.discard(item) -- Removes item from s. If item is not a member of s, nothing happens.

All these operations modify the set s in place. The parameter t can be any object that supports iteration.

Answer 6

print x,y

and you see they both point to the same set:

set([1, 2, 3, 4, 5, 6]) set([1, 2, 3, 4, 5, 6])