In [18]: class B:
    ...:     def __b(self):
    ...:         print "asdf"
    ...:

In [19]: b = B()

In [20]: b.__b()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-20-1d5710fced7a> in <module>()
----> 1 b.__b()

AttributeError: B instance has no attribute '__b'

In [21]: dir(b)
Out[21]: ['_B__b', '__doc__', '__module__']

In [22]: b._B__b()
asdf