Python's super(), abstract base classes, and NotImplementedError

Question

Abstract base classes can still be handy in Python. In writing an abstract base class where I want every subclass to have, say, a spam() method, I want to write something like this:

class Abstract(object):
    def spam(self):
        raise NotImplementedError

The challenge comes in also wanting to use super(), and to do it properly by including it in the entire chain of subclasses. In this case, it seems I have to wrap every super call like the following:

class Useful(Abstract):
    def spam(self):
        try:
            super(Useful, self).spam()
        except NotImplementedError, e:
            pass
        print("It's okay.")

That's okay for a simple subclass, but when writing a class that has many methods, the try-except thing gets a bit cumbersome, and a bit ugly. Is there a more elegant way of subclassing from abstract base classes? Am I just Doing It Wrong?

Answer 1

You can do this cleanly in python 2.6+ with the abc module:

import abc
class B(object):
    __metaclass__ = abc.ABCMeta
    @abc.abstractmethod
    def foo(self):
        print 'In B'

class C(B):
    def foo(self):
        super(C, self).foo()
        print 'In C'

C().foo()

The output will be

In B
In C

Answer 2

Do not write all that code. Simple inspection of the abstract class can save you writing all that code.

If the method is abstract, the concrete subclass does not call super.

If the method is concrete, the concrete subclass does call super.

Answer 3

The key point to understand this is super() is for implementing cooperative inheritance. How the classes cooperate is up to you the programmer. super() is not magic and does not know exactly what you want! There is not much point in using super for a flat hierarchy that doesn't need cooperative inheritance, so in that case S. Lott's suggestion is spot on. Subclasses of Useful may or may not want to use super() depending their goals :)

For example: Abstract is A. A <- B, but then you want to support insertion of C like so A <- C <- B.

class A(object):                                                                                         
    """I am an abstract abstraction :)"""
    def foo(self):
        raise NotImplementedError('I need to be implemented!')

class B(A):
    """I want to implement A"""
    def foo(self):
        print('B: foo')
        # MRO Stops here, unless super is not A
        position = self.__class__.__mro__.index
        if not position(B) + 1 == position(A):
            super().foo()

b = B()     
b.foo()

class C(A):
    """I want to modify B and all its siblings (see below)"""
    def foo(self):
        print('C: foo')
        # MRO Stops here, unless super is not A
        position = self.__class__.__mro__.index
        if not position(C) + 1 == position(A):
            super().foo()

print('')   
print('B: Old __base__ and __mro__:\n')
print('Base:', B.__bases__)
print('MRO:', B.__mro__)
print('')
# __mro__ change implementation
B.__bases__ = (C,)
print('B: New __base__ and __mro__:\n')
print('Base:', B.__bases__)
print('MRO:', B.__mro__)
print('')
b.foo()

And the output:

B: foo

B: Old __base__ and __mro__:

Base: (<class '__main__.A'>,)
MRO: (<class '__main__.B'>, <class '__main__.A'>, <class 'object'>)

B: New __base__ and __mro__:

Base: (<class '__main__.C'>,)
MRO: (<class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>)

B: foo
C: foo