Puzzling scope behaviour

Question

I can't seem to understand what is going on here:

class testclass: 

    def __init__(self): 
        print "new instance" 
    myList=[] 

if __name__ == "__main__":  

    inst1=testclass() 
    inst1.myList.append("wrong") 

    inst2=testclass() 
    inst2.myList.append("behaviour") 

    print "I get the",inst2.myList

The output is:

new instance
new instance
I get the ['wrong', 'behaviour']

I would have expected that the list in inst1 knows nothing about the list in inst2, but somehow it looks like the scope of myList trascends the instantiation of the class. I find this very unsettling and puzzling, or am I missing something here?

Thanks!

Answer 1

The way you defined myList is a class attribute.

The behaviour your looking for is the one of an object attribute:

class testclass:
    def __init__(self): 
        print "new instance"
        self.myList = []

Let's try it:

>>> t1 = testclass()
new instance
>>> t2 = testclass()
new instance
>>> t1.myList.append(1)
>>> t2.myList.append(2)
>>> t1.myList
[1]
>>> t2.myList
[2]

---

If you're interested in class attributes take a look at the Class documentation. Since classes in Python are objects too, like (almost) everything in Python, they can have their own attributes.

Answer 2

The way you declared myList inside the class makes it a class attribute. if you intended to have an instance attribute, declare it like this and it will have the expected behavior:

class testclass:
    def __init__(self): 
        print "new instance" 
        self.myList=[]

Answer 3

Yes, because that is what class attributes are for. If you want an instance variable, you need to declare it on the instance itself - usually with self inside a method.

Answer 4

myList is initialised at class instantiation time, because it is declared in the body of the class, not at object instantiation time.

Such properties are then shared with instances, until a variable with the same name is created on the instance.

So, in your case, you are using each object to access the same myList object (and append a value to it).

Answer 5

class testclass:

    def __init__(self):
        self.myList=[]
        print "new instance"

Answer 6

If you want myList to be an instance variable, you should define self.myList inside the init function. Then you should get the behavior you expect. As you have it now, I think myList is a class variable, and will be shared by all instances of the class.