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

Question

I'm working on a kind of lib, and for a weird reason i have this error.

• Here is my code. Of course @abc.abstractmethod have to be uncommented
• Here are my tests

Sorry couldn't just copy and paste it

I went on the basis that the code below works

test.py

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") 

In the python shell

>>> from test import * >>> s = SubClass() whatever 

Why for my roster module i'm having this error

Can't instantiate abstract class Player with abstract methods _Base__json_builder, _Base__xml_builder 

Thanks in advance

Answer 1

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 .