Question

Im a bit confused about inherited instance variables in ABCs. I have written an example to show my confusion. Class A needs a list which class B inherits but it must be an instance object rather than a class object. However class B also needs its own instance variable local. Can anyone set me straight?

#!python
from abc import ABCMeta, abstractmethod, abstractproperty
import unittest

class A(object):

    __metaclass__ = ABCMeta
    _internal = ['initialized']

    @property
    def internal(self):
        return self._internal

    def get_a(self):
        return self._internal

    @abstractmethod
    def set_a(self, value):
        pass

class B(A):   
    def __init__(self):
       self.local = 'OK'


    def get_local(self):
        return self.local

    def set_a(self, value):
        self._internal.append(value)


class TestCase(unittest.TestCase):

    def test_implementation(self): 
        self.assertEqual(['initialized'], B().get_a() )  # this passes but for wrong reason
        b_used = B().set_a('used') 
        b_unused = B()

        print "b_used.get_a() should return ['initialized','used']"
        print "b_unused.get_a() should return ['initialized']"
        print "b_used.get_local() should equal b_unused.get_local() = 'OK'"

        self.assertEqual(['initialized'], b_unused.get_a())  # >> fails with ['initialized'] =! ['initialized', 'used']
        self.assertNotEqual(b_unused.get_a(), b_used.get_a())


if __name__ == "__main__":
    unittest.main()

The problem is that _internal is a class obj of class A. I need it to be an instance object of class B.

Thanks In advance

Answer 1

You should initialize instance attributes in __init__() and call the base class __init__() in B:

class A(object):
    __metaclass__ = ABCMeta
    def __init__(self):
        self._internal = ['initialized']
    ...

class B(A):   
    def __init__(self):
        A.__init__(self)
        self.local = 'OK'
    ...

You should also fix your unit test:

class TestCase(unittest.TestCase):

    def test_implementation(self): 
        self.assertEqual(['initialized'], B().get_a() )  # this passes but for wrong reason
        b_used = B()
        b_used.set_a('used') 
        b_unused = B()
        ...