How do I avoid having Python class data shared among instances?

Question

What I want is this behavior:

class a:
    list=[]

y=a()
x=a()


x.list.append(1)
y.list.append(2)
x.list.append(3)
y.list.append(4)

print x.list
    [1,3]
print y.list
    [2,4]

of course, what really happens when I print is:

print x.list
    [1,2,3,4]
print y.list
    [1,2,3,4]

clearly they are sharing the data in class a. how do I get separate instances to achieve the behavior I desire?

Answer 1

You declared "list" as a "class level property" and not "instance level property". In order to have properties scoped at the instance level, you need to initialize them through referencing with the "self" parameter in the __init__ method (or elsewhere depending on the situation).

You don't strictly have to initialize the instance properties in the __init__ method but it makes for easier understanding.

Answer 2

You want this:

class a:
    def __init__(self):
        self.list = []

Declaring the variables inside the class declaration makes them "class" members and not instance members. Declaring them inside the __init__ method makes sure that a new instance of the members is created alongside every new instance of the object, which is the behavior you're looking for.

Answer 3

Yes you must declare in the "constructor" if you want that the list becomes an object property and not a class property.