Can't instantiate abstract class ... with abstract methods

Your issue comes because you have defined the abstract methods in your base abstract class with __ (double underscore) prepended. This causes python to do name mangling at the time of definition of the classes.

The names of the function change from __json_builder to _Base__json_builder or __xml_builder to _Base__xml_builder . And this is the name you have to implement/overwrite in your subclass.

To show this behavior in your example -

>>> import abc>>> import six>>> @six.add_metaclass(abc.ABCMeta)... class Base(object):...     @abc.abstractmethod...     def __whatever(self):...             raise NotImplementedError...>>> class SubClass(Base):...     def __init__(self):...             super(Base, self).__init__()...             self.__whatever()...     def __whatever(self):...             print("whatever")...>>> a = SubClass()Traceback (most recent call last):  File "<stdin>", line 1, in <module>TypeError: Can't instantiate abstract class SubClass with abstract methods _Base__whatever

When I change the implementation to the following, it works

>>> class SubClass(Base):...     def __init__(self):...             super(Base, self).__init__()...             self._Base__whatever()...     def _Base__whatever(self):...             print("whatever")...>>> a = SubClass()whatever

But this is very tedious , you may want to think about if you really want to define your functions with __ (double underscore) . You can read more about name mangling here .