Inexplicable behavior in simple example of a set in a class

Steven D'Aprano steve+comp.lang.python at
Sun Jul 3 04:25:29 CEST 2011

Saqib Ali wrote:

> I have written two EXTREMELY simple python classes. One class
> (myClass1) contains a data attribute (myNum) that contains an integer.
> The other class (myClass2) contains a data attribute (mySet) that
> contains a set.
> I instantiate 2 instances of myClass1 (a & b). I then change the value
> of a.myNum. It works as expected.
> Then I instantiate 2 instances of myClass2 (c & d). I then change the
> value of c.mySet. Bizarrely changing the value of c.mySet also affects
> the value of d.mySet which I haven't touched at all!?!?! 

But that is wrong -- you HAVE touched it. Look carefully: in myClass2, you
have this:

class myClass2:
    mySet = sets.Set(range(1,10))
    def clearSet(self):

mySet is a class attribute, shared by ALL instances, and mySet.clear()
modifies it in place. This is exactly the same as this snippet:

>>> a = set([1, 2, 3])
>>> b = a
>>> b.clear()
>>> a
>>> b

In Python, attributes assigned in the class scope are shared between all
instances. Attributes assigned directly on self are not:

class Test:
    a = set([1, 2, 3])
    def __init__(self):
        self.b = set([1, 2, 3])

>>> x = Test()
>>> y = Test()
>>> x.a is y.a  # The same set is shared by both instances.
>>> x.b is y.b  # Each instance gets its own set.

So why does myClass1 behave differently? Simple: look at the clear method:

class myClass1:
    myNum = 9
    def clearNum(self):
        self.myNum = 0

It assigns a new attribute, rather than modifying the object in place!
Assignment to self.myNum creates a new unshared attribute:

>>> z = myClass1()
>>> z.myNum
>>> z.__dict__  # No instance attributes yet.
>>> z.clearNum()
>>> z.__dict__  # Now there is one.
{'myNum': 0}
>>> z.__class__.myNum  # the class attribute still exists

The simplest way to fix this is to move the declaration of self.mySet into
the __init__ method.


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