List in a Python class shares the same object over 2 different instances?

I created a class:

class A:
    aList = []

now I have function that instantiate this class and add items into the aList.

note: there are 2 items

for item in items:
    a = A();
    a.aList.append(item);

I find that the first A and the second A object has the same number of items in their aList. I would expect that the first A object will have the first item in its list and the second A object will have the second item in 开发者_如何学运维its aList.

Can anyone explain how this happens ?

PS:

I manage to solve this problem by moving the aList inside a constructor :

def __init__(self):
    self.aList = [];

but I am still curious about this behavior

You have defined the list as a class attribute.

Class attributes are shared by all instances of your class. When you define the list in __init__ as self.aList, then the list is an attribute of your instance (self) and then everything works as you expected.

You are confusing class and object variables.

If you want objects:

class A(object):
    def __init__(self):
        self.aList = []

in your example aList is a class variable, you can compare it with using the 'static' keyword in other languages. The class variable of course is shared over all instances.

This happened because list is a mutable object, and it is created once only when defining the class, that is why it becomes shared when you create two instances. Eg,

class A:
    a = 0 #immutable
    b = [0] #mutable

a = A()
a.a = 1
a.b[0] = 1

b = A()
print b.a #print 0
print b.b[0] #print 1, affected by object "a"

Therefore, to solve the problem, we can use constructor like what you have mentioned. When we put the list in constructor, whenever the object is instantiated, the new list will also be created.

In Python, variables declared inside the class definition, instead of inside a method, are class or static variables. You may be interested in taking a look at this answer to another question.