gistfile1.txt

In [1]: class C():
   ...:     def __x(self):
   ...:         return 0
   ...:

In [2]: c = C()

In [3]: dir(c)
Out[3]:
['_C__x',
 '__class__',
 '__delattr__',
 '__dict__',
 '__dir__',
 '__doc__',
 '__eq__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__gt__',
 '__hash__',
 '__init__',
 '__le__',
 '__lt__',
 '__module__',
 '__ne__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__',
 '__weakref__']

In [4]: c._C__x()
Out[4]: 0