Python object instance inheriting changes to parent class by another instance

[PyNoob123]

I am confused by this behaviour of Python(2.6.5), can someone shed light on why this happens?

class A(): 
    mylist=[]  
 
 
class B(A):
    j=0
    def addToList(self):
        self.mylist.append(1)
     

b1 = B()
print len(b1.mylist)  # prints 0 , as A.mylist is empty
b1.addToList()
print len(b1.mylist)  # prints 1 , as we have added to A.mylist via addToList()

b2 = B()
print len(b2.mylist)  # prints 1 , not 0 !!! Why ?????

[bvdet]

The list is initialized one time when class A is defined. When instance method addToList() is called and no attribute named muylist exists in the local scope, the interpreter finds mylist in the scope of A and appends 1 to it. If you want a new list created every time you create an instance, implement an __init__() function.

Example:

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

class B(A):
    def __init__(self):
        A.__init__(self)
    
    def addToList(self):
        print self
        print self.__dict__
        self.mylist.append(1)

Some interaction:

>>> b1 = B()
>>> b1.mylist
[]
>>> b1.addToList()
<__main__.B instance at 0x00F6E030>
{'mylist': []}
>>> b1.mylist
[1]
>>> b2 = B()
>>> b2.addToList()
<__main__.B instance at 0x00F6E6E8>
{'mylist': []}
>>> b2.mylist
[1]
>>>

[MojaveKid]

mylist is a class variable and this is shared by all A (instance or not)