Are Python sets mutable?

问题:

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 y still pointing to the same object, or was a new set created and assigned to y?

回答1:

>>>> 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])

Conclusion: sets are mutable.

回答2:

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).

回答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

回答4:

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])

回答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.

回答6:

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