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 .