I didn't get it yet!

sttr.py

class A(object):
    __a = 10

a = A()
print dir(a)

"""
    should return something like: 

['_A__a', '__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']


Realize that your variable _a is renamed to _A__a
"""

class B(object):
    def __init__(self):
        self.__b = 10

b = B()
print dir(b)

"""
Again, our variable __b is renamed to _B__b

['_B__b', '__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']
"""

class C(object):
    def __init__(self):
        setattr(self, '__c', 10)

c = C()
print dir(c)

"""
But in this case, __c is __c instead of _C__c:

['__c', '__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']
"""