Question

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 !!! Why ?????

Answer 1

You need to do:

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

That way self.mylist is an instance variable. If you define it outside of a method it is a class variable and so shared between all instances.

In B if you define a constructor you’ll have to explicitly call A’s constructor:

class B(A):
    def __init__(self):
        A.__init__(self)

This is explained (not very clearly) in the Python tutorial.