Are Python Sets Mutable?

Answer :

>>>> 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.

Your two questions are different.

Are Python sets mutable?

Yes: "mutable" means that you can change the object. For example, integers are not mutable: you cannot change the number 1 to mean anything else. You can, however, add elements to a set, which mutates it.

Does y = x; y |= {1,2,3} change x?

Yes. The code y = x means "bind the name y to mean the same object that the name x currently represents". The code y |= {1,2,3} calls the magic method y.__ior__({1,2,3}) under the hood, which mutates the object represented by the name y. Since this is the same object as is represented by x, you should expect the set to change.

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You can check whether two names point to precisely the same object using the is operator: x is y just if the objects represented by the names x and y are the same object.

If you want to copy an object, the usual syntax is y = x.copy() or y = set(x). This is only a shallow copy, however: although it copies the set object, the members of said object are not copied. If you want a deepcopy, use copy.deepcopy(x).

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.